In most cases (with exception large or high profile acquisitions) they do. However, the process is much less selective (i.e., most -- but not always all -- engineers in the acquired company get an offer) then if you apply directly to Google. Those who do not get an offer get a generous severance package.
I think it's more that the risk of a no-hire is spread across the whole company rather than it being less. If a potential acquisition target has a critical mass of lackluster employees, Google (and presumably other large companies) simply won't buy the company. So while the bar for you personally may be lower if the rest of your coworkers are good, the bar for you as a group is just as high or higher than individual hires.
