I guess they could create a system where you develop a reputation as an infamous thief even if you don't get caught on the spot.
Hostile guards and merchants should make your life more difficult in the long run.
I am afraid it is one of those Larian's "it breaks the game but is FUN" things. Merchants have been a joke since D:OS1.
I suppose to some it is "if you don't like it, don't use it", to me: "can't roleplay as a thief or the game falls apart".
Also for me it's the opposite of fun because it makes playing a thief a cheap and tired exploit that is not satisfying or rewarding in any way.