Rocket engines is what they're buying in this instance (or rather, what ULA has been prevented from buying by the injunction): specifically, RD-180 engines, built by the Russian firm NPO Energomash for use on the first stage of United Launch Alliance's (otherwise) American Atlas V.
(BTW, quite a few of the rocket makers you named buy their engines from others. ULA also buys engines from Pratt and Whitney, and Orbital has several suppliers for its various launchers.)
The larger suit does concern a block purchase of whole rockets from ULA, but the injunction does not interfere with the block buy, just payments to the Russian subcontractor.
Interesting, ok so what special requirements could there be attached to Russian engines that US built engines could not supply?
I can see how if they are dimensioned in such a way that they will fit only a launch vehicle by a single party how that might be usable as a trick to lock out competitors (first order launch vehicles without engines, then order engines without launch vehicles). Both of those would be harder to compete with than ordering whole rockets!
Among kerosene/oxygen rockets, the RD-180 gets an unusually high amount of propulsive kinetic energy per unit of fuel (technically, high specfic impulse), in part due to a more efficient combustion cycle (staged combustion), and in part due to very advanced materials science the Russians have, that the US hasn't matched. (And other engines with similar performance are also Russian: the RD-170, which is signficantly larger than the RD-180; the NK-33, which is quite a bit smaller...)
There is an American manufacturer that has licensed the RD-180 design, and technically has the rights to build them here, but they haven't --- and since that would involve duplicating Russian metallurgy, there's some skepticism that they could. (At a recent Senate hearing, it was the head of ULA(!) who said the Russian design did "things we found that our textbooks said were impossible".)
If you're asking why ULA can't just buy something else from, say, Pratt and Whitney, then the short answer is, "they don't sell anything that fits on an Atlas V". Think about trying to use the engine from a VW bug (or a large truck) to replace the engine in a pickup: even if the fuels are the same (and you can't take that for granted!), the fittings and performance characteristics are different enough that the combined thing just wouldn't work.
The Chinese are developing an engine, the YF-100, based on the Russian RD-120. It's a staged combustion engine with an oxygen-rich preburner -- very similar technology to the RD-180.
The YF-100 has already gone through a full-length test firing on the ground, and the CZ-7 rocket that it's mounted on is scheduled for a test flight later this year.
If the Chinese have been able to make it, then I see no reason why we can't.
P.S. Does Pratt & Whitney still make rocket engines? I thought they sold Rocketdyne to Aerojet?
About the engines: heck, the Space Shuttle Main Engine was staged-combustion Oxygen-Hydrogen. The question isn't whether the US could produce such an engine. It's how soon, and how reliably. ULA has laid in what they say is a two-year stock of engines as a hedge against ... hiccups in the supply chain, but they'd need new engines after that. For a clean-sheet design, that would be a crazy aggressive schedule. With RD-180 design docs in hand, well... we may soon see.
In a hydrogen engine, you can do fuel rich staged combustion.
With kerosene, you must go oxidizer-rich. Nobody has mastered that except Russia. Nobody really knows how much it costs, or how they came up with it.
SpaceX uses the far simpler gas generator cycle.
In staged combustion, you pump some of the propellants to some absurd pressure like 300 bars, burn them, put them through a turbine that extracts some power that lowers pressure to 250 bars or so. Then the turbine exhaust is put into the main combustion chamber, together with the other propellants that were pumped up to 250 bars. All propellant contributes to thrust.
In a gas generator, you pump all propellants to perhaps 70 bars, divert some small part to a gas generator that burns them, the exhaust is put to a turbine that exhausts to ambient air. The turbine powers the pump. So that part of the propellant flow does not contribute directly to thrust.
If you go to higher pressures with a gas generator engine, you waste a larger portion of the propellant to run the pump and it's just not worth it. So gas generator engines are limited to much lower pressures than staged combustion engines.
Lower pressure means bigger engines are needed for the same thrust. Also the efficiency is worse. A large (tall) rocket can have problems with limited base area per mass, hence for example the low pressure engined Saturn V had an expanded base.
Space Shuttle's main engines ran with high pressure and could fit on the back of the relatively small reusable orbiter vehicle.
(BTW, quite a few of the rocket makers you named buy their engines from others. ULA also buys engines from Pratt and Whitney, and Orbital has several suppliers for its various launchers.)
The larger suit does concern a block purchase of whole rockets from ULA, but the injunction does not interfere with the block buy, just payments to the Russian subcontractor.