The problem is still the huge percentage of American traffic on the web. E.g. on Iconfinder.com about 50% are US users, so blocking them would hurt ad income by about the same amount.
The US population is roughly 11% of all internet users.
If the current political trends continue to the point it jeopardizes my business, and creates unnecessary risk, I would have no problem blocking ±10% of my users, right then and there.
I sell a Windows app. Americans may be only 11% of the population, but they're about 50% of my customer base. They buy licenses, which gives me incentive to continue working on the software. The Russian and Indian bloggers (for example) who have reviewed my app positively have moved the needle very slightly, if at all. No chance I'd give up my US customers willingly.
But that's also changing - North America had the least growth in internet usage from 2000-2011: http://www.internetworldstats.com/stats.htm