>Every job application I've seen asks if they may contact past employers

Yes. They ask this as a professional courtesy. And if you look closely many times they are only asking about your CURRENT employer because people understandably don't want to be put in a bind at their current job because they tested the waters on a new one. An employer looking to hire doesn't want you to have reason to be afraid to apply to them. Neither restriction is required by law. It's just good form and mutually beneficial to both parties.

>and you must give written permission for any CORI checks

CORI and criminal background history in general is COMPLETELY separate from employer history.

You're inferring a connection that does not exist between these two things.