It's a bit more complicated.
Actually the jailbreak works to Apple's advantage.
With the carrier-locked firmware it can pay lip service to its agreements with AT&T or insert_local_carrier_name.
Without jailbroken phones there wouldn't be much (prolonged) international interest, until available at that particular country. People would have to go with something else, because it wouldn't be an option. Jailbreak has fed the frenzy and did not cut into Apple's profit, quite the reverse in fact. And since it is not sanctified by Apple, it has no responsibility - best of both worlds. It certainly is not like PSP firmware hacks which -among other things- enable running pirated games, which does cut into Sony's income (Sony receives a share of all games sold, just like MS and Nintendo).
iPhone is a sophisticated piece of hardware, with a very good UI. But I don't consider it a smartphone unless it can run pda-style 3rd party native applications. SDK may be out, but without jailbreak and no apps at iTunes store it is still a glorified ipod with a very good browser. This will change in the near future, I am talking about right now. Capable and stylish: yes, smartphone: no. Not yet.
My el-cheapo dumbphone can read email and browse the web too. But I cannot add a new SMS app or change the phone front-end, just like iPhone. This is something I was easily able to do on my Treo.
The new SDK may be good enough for games, but without allowing multitasking or access to the calendar/email/sms/phone/etc, it may be too limited. No 3rd party email/im app running in the background, for instance. Or a decent todo list that works with the calendar. Even with the SDK, jailbreak will still be the only way stuff like this is possible.
The problem with jailbreak is no commercial company can count on it to create software it can sell. You cannot ask your customers to void their warranties to run your application.
When Apple begins to receive a share of all s/w purchased, jailbreaking may become harder. On the other hand history has shown almost all protection schemes will be bypassed, one way or the other..
P.S: the iTunes store is still not available in many countries.