It is probably inconsistent that the views behave differently, however to be more flexible I think this is part of an oft-repeated request that you should be able to display whatever images you want based on the current value of the folder name. The problem is recognised, it's just that no-one's got an answer for it and the JRiver team won't reply.
It is widely recognised that artist images only work on the Artist tag and not Album Artist (not just in Theater View but in Gizmo, JRemote, eos, Web Gizmo, DLNA controllers etc) which is rather silly for an album view. You should be able to say in the category definition that you want to use the Album Artist tag to get the images. BUT instead you might have a Sort Artist field that you like to group your views by, so a harcoded Album Artist won't work for that, in the same way that the hardcoded Artist tag doesn't work currently. You should be able to specify the Sort Artist tag for that folder.
And this same mechanism would then work for all other tags, e.g. you could have custom images for your Genres, stored in a Genres images folder, for example. If you do a search for this (artwork/cover art at folder or category level), you will find many requests for it over the years.