The main reason for it is not having questions about things broken in betas, but now that they're public, and people are installing them on their main devices (you know they are), they're valid questions with valid workarounds as answers.

Heck, I have iOS 9 on the phone I carry in my pocket all the time. Bad idea? Maybe for some folks. But I can deal with a few bugs.

Another reason for having that ban is to keep from having questions about things that are now fixed, but Stack Overflow seems to deal with it just fine, and Google knows what they're doing. If anything, make a new off-topic reason "This issue has been fixed in a newer (beta or final) version of the software. If you are having this problem on a recent public release, ask a new question"

(or whatever you want them to do)

And the legal aspect about it isn't relevant, we shouldn't be enforcing legal agreements between two other parties anyway.

Sometimes it seems like half of the challenge of helping people, is getting them to actually check back and see your response. So often answers seem to just fade into the abyss...