It does need to be a protection racket though. Mullenweg didn't walk up to WP Engine and say, "sure would be a shame if you somehow lost access to the plugins repo, if you pay me some $$$ I'll make sure that doesn't happen." He wrote a public blog post about how they turn off features he considers essential to WordPress. I'm not him, but it seems like if they just flipped that feature on (which would enrich him not at all) he'd be cool.