Amazon is already leaving Texas. Or, at least, they are closing their distribution center there. See this article (or just google it). They are presently building new facilities in Tennessee near Charleston (about 10 miles away from me at the moment) and Chattanooga. There is already talk in the Tennessee state legislature about passing a new law expressly to renege on the sales tax exemption granted as a condition to building the distribution centers here. Just goes to show that all state representatives are clueless -- it's not limited to California.

As some have already mentioned, collecting sales taxes cross-state is a significant burden since, in some states, the rates may differ even from one side of a street to another. Using ZIP codes does not provide enough granularity to determine the proper rate. With this in mind, several states started the "streamlined sales tax project" which aims to provide the data for determining the proper rate, a single point of reporting and indemnifying businesses from errors in the rates supplied. If every state which imposed a sales tax adopted this system, it would practically eliminate the burden facing Internet (and traditional mail-order) businesses today.

That said, enforcing the use of this system would require Federal legislation and, even then, there will still be the issue of purchases from other countries. I'm not so sure that it's a good idea to get the Federal government involved anyway since they might be too tempted to add a Federal sales tax as well.

There already are Truth in Advertising laws.

It's been my experience that, when you think "there ought to be a law ...", there already is one. The problem is the law being either selectively enforced or not enforced at all. And, typically, the more a law favors the public over corporations, the less likely it will be enforced.

But ... IANAL (so what do I know?)