The problem here is that if you do that, then you've just destroyed the tenure system. Once you have postdocs having long term contracts, and getting into positions of authority, then pretty soon, that's going to be the standard practice. One thing about legislation is that when it comes to politicians, both post-docs and tenured faculty are on the same side. No one wants politicians to start mandating terms of employment.
I'm reminded of the conversation between Bryant and Deckard in Bladerunner in which it's mentioned that androids why have a programmed four year life span.
One other thing to note is that tenure was not unusual in most unionized industries in the 1950's. It's only because of changes in the labor market that made professors have different hiring practices.