why pick on the thieves,the only role they play in the entire game is just that..thieving!
pinning them back with more hurdles takes away their speciality whether its in small teams or large.
Restricting their numbers relative to the size of the town/city they attack, might be more appropriate, ie that the larger the town the more you can send would be fine, having to keep sending groups of thieves out in small numbers is just simply making too much work on the keyboard, not mention very tedious.
restrictions on size of attacking thieve armies relative to town targets would also safeguard the smaller town without killing the concept of using thieves.
but TBH why restrict them at all? this is a game of all sorts, there will always be victims and besides, from what ive seen in the GC, people always gang up and track any would be perpertrators and get their justice one way or another as it is.