Well, the valuation of companies do include IPR, so in that sense it isn't entirely illegal to get hold of the patents owned by a company when they take it over. What is probably needed is regulation over what is a patent and what is not. And what is an acceptable duration for holding the rights. I don't remember exactly, but in the US there are 3 durations - 3, 7 and 10 years. After that it comes up for renewal