No, a bill such as this won't endanger publishing companies... publishing companies have endangered themselves by pissing off their customers with insanely high pricing.
No, they haven't. Every attempt to secure the cheaper alternative for textbooks fails for one of several reasons:
1. Planned obsolescence of the original edition (see the posts above me for a more elaborate explanation of this). In particular, the changing of textbook problems and the unwillingness of the professor to use an alternative way to assess students (such as custom problems written by the professor himself/herself), as well as online stuff that only works once and therefore kills any chance of resale of the textbook. regardless of its edition status.
2. So-called "cheap" textbook sites not having much difference at all in the price of the textbook verses that set in the bookstore, or even in some cases actually costing more than what the bookstore charges.
3. The unwillingness of the bookstore to make any real difference on pricing of used and new books, buying back books for less than 20% of what they cost new, or even outright refusing to buy or sell used books entirely.
The result: while theoretically students have a choice in where to purchase textbooks, in practice this is irrelevant because outside factors mandate the latest edition, and most places seem to charge the same amount.