While your intentions are undoubtedly admirable, and you certainly more than likely have very valid reasons for not "pulling crap like this" (not burning bridges, for one), I hope you're not mistaken by some baseless belief that any company would extend you the same courtesy.

Meh.

I think it's pretty rare for a company to tell someone "Yes, you can definitely work for us, here is a contract for you to sign and a start date and how much you're going to make" and then revoke that.

Which is the equivalent to me telling a company I'll work for them and then reneging.

But of course, yes, they could do it, and you're right that people and companies aren't always courteous.

I've had a company retract an offer after I asked for an extension to accept. I guess they felt like a second choice.

I'm glad your experiences have been so plush. In my past as a graphic designer (this doesn't happen anymore in web development, mind you), I've been hired and then let go in the span of a few months, due to cuts and layoffs. Maybe I'm burned and a bit cynical, but the lesson I've taken away is that business is business.

It's not "pulling crap", as you say. For one thing, until you sign the contract, you're not bound in any way. Even then, the first few months are typically defined as a trial period, in which both parties are free to walk away. Better opportunities always crop up, and you can just say "sorry, a better offer has popped up".