Shockingly, it's a bit more complicated than that.
There are, in fact, cases where people have successfully argued that enacting regulations on previously owned assets constitutes a taking. This has been litigated with regard to your example of zoning regulations.
The Supreme Court most recently ruled on this area of law last month, in Stop the Beach Renourshment vs FDEP http://www.law.cornell.edu/supct/html/08-1151.ZS.html. In this case, the Court ruled that courts, not just executive or legislative authorities can in fact engage in lawful takings- and therefore require renumeration.
In Lucas v. South Carolina Coastal Commission (1992), the court refined a number of earlier decisions going back to at least 1926. Lucas established a very narrow interpretation of takings- in that all economic use of a property had to be restricted. There are, again shockingly, different opinions as to whether this established this as the only test or was a minimum. A more recent case is working its way through the system that tests the Lucas standard, and compares it to the earlier "Penn Central" balancing test which weighs- "the economic impact of the regulation on the claimant, and, particularly, the extent to which the regulation has interfered with distinct, investmentbacked expectations,” and “the character of the governmental action”.
In short, anyone who thinks this is obvious is letting their opinions get in the way of their analysis of the law and Court opinions. The Supreme Court has demonstrated in "Stop the Beach" that it is interested in moving to greater restrictions on compensation rights for those lawfully taken from, and the Penn Central balancing test would seem to be something that could reasonably be argued to require compensation for economic loss caused by restricting the use of an asset.
As a political matter, whether the government has the authority to do something or not should not be the only test we employ. We should consider the effects of the action on the general economy as well as the fairness of those effects.
This becomes very clear when you look at situations like the Kelo case- a taking by government of private land (with compensation) for use by another private entity. The court ruled this was acceptable, but most people think this is an unreasonable thing to do. That particular case makes a strong point of the damage that government authority can do when applied poorly- the Kelo house was moved, at great cost and legal effort, and the land now is an empty field, undeveloped- in short nothing useful occurred, at great expense. The government had the right to do something, but it was a bad decision.
Closer to this issue is my own example. I started working on the Internet in '91- before the Mozilla browser existed. I spent almost all of the '90's working at building, re-architecting, merging and running ISP's and NSP's. I can attest that coming up with a definition of what we were selling, particularly on leased lines that were equivalent to a significant portion of our upstream capacity was a frequent point of discussion and effort. I also will tell you that I got out of that business as I started to see that various regulatory forces were coming to bear. I didn't want to be part of a market where winners and losers get chosen, even in part, by political considerations.
Now, without being to cocky, I think most observers would say that I've contributed to the creation of quite a bit of economic prosperity in the last 20 years. I did this, despite changing the direction of my career, in large part because of incoming regulations.
How many people like me jumped out of that business? I know a few others personally (it was, and still is, a rather libertarian crowd).
Lets say it was 5% of the top 200 engineers running the various ISP's at the time. What was the economic impact of that? What was the impact on the development of the Internet?
My point, such as it is, is this- the legal environment on this area is actually vague. Beyond that, the impacts of regulations to peoples lives, fair play and the economy can be more significant than expected. I for one prefer to give great deference to individuals liberties and reasonable expectations that the rules of the game won't be changed on them.
This isn't just a simple argument over what "makes sense" to you or me. It's an argument over whether what makes sense to some people, perhaps a majority, should take away some limited amount of economic freedom from others- particularly when they have invested based on expectations of that freedom remaining. I for one, am having a hard time seeing that standard met in this issue.