I'm guessing that most acquihires deal have a clause preventing you from leaving the company ASAP. Or at least an incentive to make you stay with the company (shares, yearly bonus, etc.). I don't think google will buy a company XM$ and see them leave right after.

Another point is that the acquiring is not "hostile". This is a deal between the startup/team being acquired and the large company so they weighed their options and chose to accept the offer - i.e. they want to work for the big company or are interested in the project they are being offered.

There is a lot of threads here on HN describing that talent is hard to find in technology and it's understandable to acquire a team that already work well together and produced something concrete.

One last thing was a comment by pg here : http://news.ycombinator.com/item?id=4366621 saying that "The article doesn't mention one of the most important reasons companies do HR acquisitions: competition forces them to. If company A offers to acquire a startup and company B merely offers to hire the founders, all other things being equal the founders will take company A's offer."