It is recommended that you enforce TLS so that when you send/receive between your On-Premise & Cloud Recipients the communication is secure, just like if it was internal. You wouldn't allow mail to be routed around your internal infrastructure unsecure would you? so why do it in this scenario.
I would look into the reasons as to why you have an issue using TLS, I would imagine it's something to do with the SSL Certificate being used as in FOPE it will have a rule which states...
The Security.. which generally is setup as follows (certificate Subject Name Matches X) Attachment 20879
and it routes email via smarthost (which points to the Public IP of your Hybrid Server) Attachment 20880
I agree but it's down to the company if they want to send via TLS. There 10,000 companies I know who still use SMTP without TLS. Most finance sectors do as its a requirement but many O365 customers still don't.