A lot seems to have to do with the type of rental agency and who their target market is. For example if you rent from one of the "top tier" agencies (Hertz, Avis, National), you'll find the least number of restrictions. Their target market is the business traveler, and those people want a "no hassle" experience. This includes the rental and return procedures, as well as the restrictions in the contract. Their unlimited-mileage rates typically have a very wide allowable geographic range (such as "lower 48 US states and Canada").

The further down the food chain you go, the more restrictions and more hassle you'll get. Budget seems to be about in the middle, but if you start scraping the bottom of the barrel, you'll find the most restrictions, including geographic restrictions mentioned above even on an "unlimited mileage" rental.

As someone else already reported, the top-tier agencies typically charge a bit more, but in my opinion they're well worth it.