The SMS fallback experience in iMessage is pretty bad. Some of it may be because SMS is bad itself (the unreliability), but the way iMessage features get downgraded to SMS creates a terrible experience for everyone. Add MMS in the mix and it gets even worse. To be fair this can be ascribed to technical limitations; in Apple's view iMessage works great and it's too bad you're using something crappy like SMS/MMS. But then Apple hasn't put any effort into making the non-iMessage experience better.

As for why it's anti-trust, it's right there in the WSJ article. Apple executives are explicit in the lock-in being useful for their market power.

Also it may have been years ago but I'll never forget the way Apple originally captured phone numbers into iMessage. Once a phone number was registered in to iMessage no Apple system would ever send SMS to it again. That created a major problem if the owner of the number had stopped using Apple phones. They were sued over this lock-in and finally implemented a way to release your number from iMessage jail. It's still kind of awkward though.