I’m not aware of any studies on this, so anecdote is all I have. But for me, it’s “obvious” that politics is a factor rather than “obvious” that tourists only think of trends and exchange rates.
In fact, it’s now common (at least in my circle) for people to plan vacations to avoid changing planes in the USA, because of the delays in customs and the risks of missing connections or being denied entry to the US (required in the USA even if you never leave the transfer area).
Is it true that they must clear customs _in addition to_ passport/immigration control? If so that is indeed unusual.
If you had flown into Amsterdam and transited to Moscow you would have gone thru metal detectors to the departure lounge, but not thru immigration
I'm inclined to think it's had a chilling effect when tourists are making their holiday choices. It's pretty easy to skip the USA and find another destination.
If anything, despite some stagnation, the trend is more or less the same and still rising in 2017 and 2018 (click "5 years"):
Also some countries are now issuing travel warnings about the US because of the mass shootings, especially those aimed at visitors.
NZ - "Exercise increased caution in the United States due to the threat of terrorism." ..."There is a heightened threat of terrorism in the United States and terrorists remain likely to try to carry out attacks. A number of politically motivated attacks have occurred in recent years, causing multiple deaths and injuries."
Australia - "The United States has more violent crime than Australia, although it rarely involves tourists. Shootings, including mass shootings, can occur in public places"
UK - "Terrorists are very likely to try to carry out attacks in the USA. Attacks could be indiscriminate, including in places visited by foreigners. You should monitor media reports and remain vigilant at all times."
Here's a more general survey of such travel warnings
This also makes the idea of visiting Canada much harder, since most flights stop over in the US, causing visa headaches.