Relevant laws here: https://www.law.cornell.edu/uscode/text/17/117You’re correct.
Legally you can do whatever you want with an .ipa app. Try to execute it on any platform you want, mail it to all of your friends for all Apple cares.
Legally, Apple is allowed to prevent you from actually doing anything useful with the .ipa file if it’s being used in a manner that falls outside the license agreement.
I haven’t read the agreement close enough to know if it covers running the application on the iOS operating system... but regardless Apple is under no legal mandate to allow you to run the application wherever you want. They have to allow usage in within the bounds of the agreement. If the license states that it’s only to be used on iOS, Apple is well within their rights to prevent usage elsewhere.
I don't know what praxis is in the US, but there definitely is a possibility to argue that Apple has taken away the consumers' legal right to make adaptations to make functional previously purchased/licensed applications.
It's a very complex issue, as even the previous laws in the area have been sort of a patchwork on top off pre-digital philosophies about copyright/ownership. So there are a lot of stuff in current licenses that simply haven't had their day in court to see if they stand up or not.