The problem is the Warning is not necessary, the AR-5RM does support these frequencies/AM.
This is because the default for a new channel is FM, and when you're creating a new channel from scratch, you start with frequency and get the defaults for everything else. Most radios that support AM can do AM or FM on any frequency. However, several new models (the 5RM included) are very limited and only support AM within airband and nowhere else (and not FM on those frequencies). Since CHIRP was designed for radios without such limitations, the default workflow ends up having to warn you that you can't put an FM channel in with an AM-only (according to the radio) frequency. The warning *is* necessary because if you intentionally try to do this (i.e. change an airband/AM channel to FM or paste an airband/FM channel in), the warning makes sense. You're just getting it by virtue of the default.
The solution here is that chirp should be inferring the default/correct mode from the band according to the bandplan when setting a new channel, but for some reason we don't have airband/AM in there, so it's defaulting to FM.
This isn't really the best place to just report a behavior you don't like:
https://www.chirpmyradio.com/projects/chirp/wiki/How_to_report_issues
But if you'll file a bug for this, I'll fix it for the next build so that it behaves a little more smoothly.
--Dan