The transfer restriction is on transferring registrars, not changing registrants. You control the whois records so you can sell the domain, or change other contacts, if you wish.
If you transfer after the 60 days, yeah, they keep your $10 bucks but you also keep your registration time; when you pay for a year with your new registrar that year will be added to what ever time was left on your year’s payment to DreamHost, do you loose nothing (you just extend your registration).
This is pretty much standard, though some registrars are more restrictive. That said, you should suit yourself and register your domains wherever you want. If your registrar has reasonable DNS policies, you should have no problem hosting any domain at DreamHost no matter where it is registered.
You are not locked in at all. DreamHost will not allow registrar transfers for the first 60 days (after initial registrartion), but after that, do what you want and you shouldn’t loose any money via a transfer (depending upon your receiving registrar).
If you think 60 days is unreasonable, you can try to find a registrar that will transfer sooner than that, but that is pretty much standard practice (see below), and I do not know of any registrars that do not have a similar, or more restrictive, policy.
As for why the waiting period, I believe it is an ICANN rule (http://ttp://www.icann.org/transfers/policy-12jul04.htm) and I suspect it has a lot to do with the fact that there is so little money to be made in registration that registrars don’t want to be bothered with registering something they will have to transfer again in less than 60 days (cost them more to process the registration, and the transfer, than they would make?)