It also prevents competition.

Locking down Magneto would have resulted in a fork that could grow to rival the product Adobe now owns. With a community edition, the community has less incentive to fork. You can still use the product, and surely that will never ever change, so why bother getting to work on a competing fork?

Then years down the line, when no competition has come to exist, when the industry has grown dependent on the product, they can start strangling the community edition.