The reason web devs do this is so that their format doesn't break when Safari tries to make the text much larger than intended. Depending on how you laid out your elements, mobile Safari can render your (non-mobile) site completely broken if you don't set this CSS value, as it doesn't respect your font-size values by default, making text elements take up much more space than intended in the design. Often, it's seen as a better option to make mobile users scroll horizontally and vertically than to render the site like crap with elements spilling out of their boundaries.
I'd say it's similar to the use of a mousedown to block a right click dialog. Sure, some idiots use it to disable right click, but a lot of applications (See: about everything Google's done) use it very well for creating in-app contextual menus.
This adjustment is pretty superfluous outside of iOS though, I can't think of any non-artistic reason to use it that couldn't be solved through transforms and an understanding of how Safari scales things.
