They probably did. The industry was small. Attracting developers in significant numbers meant convincing people to become developers. That is a huge hurdle. Platforms with entry fees didn't build app ecosystems that attracted end users.
By the time the iPhone was announced, developers were plenty and the software business model proven. Apple had resources to subsidize onboarding with a well-documented, rich API and good native apps.
The difference I see is more about Apple specifically, because unlike, say, Google with Android, they have full vertical integration. They make both the hardware and the OS. Everything about Apple is focused on complete corporate control over the experience.
If Apple decided to only permit apps distributed through App Store X on OS X, I wouldn't be at all surprised. That's Apple's MO. And there is a customer segment I think that serves, which is the consumer market which doesn't want to think about maintaining their own digital security, because Apple seems like a reasonably benevolent dictator in this respect, as much as developers dislike the cut they take. Requiring Apple to open their devices to third-party app stores would break this vice grip they have on the experience and, IMO, degrade the experience for Apple's end users (even if it might make things nicer for developers).
Hard to fault a business for realizing that they have a stupid amount of leverage because people have made themselves dependent on you and charging more.
What? If a business is causing a large negative externality, that's a bad thing.
Just because you can't "fault" people for pursuing profit doesn't mean they should be allowed to do it any way possible?
I just really don't understand this worldview.
If not, what's the difference with phones/tablets?