I don't know of any viable desktop solutions for this issue, but it seems to be how iOS works. Unless you jail break your phone you cannot download or install anything not on the app store (where everything goes through a review boardI believe).

And I haven't heard much about iOS viruses really. Maybe for the jail broken ones?

A quick google search turns up this malware [0] which only works through USB. So the system does seem to be working pretty well all in all. Reducing the attack vectors from everything to direct hardware connections only is a pretty big accomplishment I'd think.

[0] - http://www.computerworld.com/article/2843764/security0/wirel...