It depends on the purpose of the acquisition. If the acquisition was to eliminate a competitor and increase market share, a lot of times the assets and personnel of the acquired company will remain (with the exception of redundant positions or overlaps) in order to handle the increased business. Why fire people that already know the ins and outs of the job, the client base, and the legal/regulatory environment for no other reason than that they were part of the acquired just so you can rehire, retrain and relearn a bunch of new employees? So while the company that was acquired may seem to "cease to exist" in reality the assets and people frequently are still there doing the same jobs but with a different name on their paychecks.

Now if the purpose of the acquisition was to break up and sell off parts of a company then thats a different story and is usually more painful for the employees involved and typically results in significant loss of jobs and shedding of assets all in the name of short-term gains for the shareholders of the acquiring company.

As far as being "forced" to take on the work force of the acquired company, I don't know for sure of any other industry that is "forced" to take on those employees. If the airline industry is truly "forced" to take on those employees through state or federal law, then I can only imagine it has to do with the necessary role air transportation plays in the national economy and transportation infrastructure. Its the same reason we can't strike at the drop of a hat. But when you think about it, forced or not, its in the acquiring company's best interest to take on those existing employees at the acquired company because the alternative is a lot of airplanes and gate space sitting idle and vacant until you can hire and train new employees. That, my friend is an expensive proposition and does little to benefit anyone.