How can this flaw be an intended design of a blank partition?

[PommeDeTerre]

If you don't trust certain members in the FC house, you always have the option to block them from Estate Hall Access in the rank settings. The other option is to bypass using expensive wall coverings on the blank partitions and expensive fish/trimmings in the aquariums. Paintings certainly aren't worth the worry since they only cost 1k gil.

Well, not letting FC members in the FC house is kinda the opposite of what I want obviously lol. It's not that I don't trust my FC members, it's that some members are new to the game and I doubt they would see harm in using an interactive object. And your other option of not using expensive furniture just because there's a chance someone will remove it is silly. SE should just fix it instead. It is not working as it should.

I keep our FC house pretty simple because it is for the entire FC and I want it to look like something that belongs to a FC, not a private designer home.

Making something original and "designer home" worthy can make your FC stand out from the rest. And members, in my experience, appreciate a nice design. If you want to keep your design simple that's entirely up to you but I don't think I should be limited in designing options just because there's a flaw in how object interaction works right now.

If I set up a rank and members in that rank have "No furnishing privileges" they shouldn't be able to change or remove wall designs, remove aquarium tank trimmings and fishes, change paintings at will and delete flowers from pots, it's as simple as that.