Why fine penniless Westerners for begging in Thailand?

Two tourists were charged with “committing trouble and annoying other people in public”, the article states.
I can understand them being arrested, but why give them a fine? Well, that I cannot comprehend!
The reason why they are begging is that they lack funds. So how are they going to pay a fine? It absolutely defies logic to make them do this.
I might also add that many Thais annoy Westerners by begging for money, or bothering them when they see foreigners simply taking a stroll, especially taxi drivers, who are always looking to make a quick buck. 
So if the police are going to arrest foreigners for being an annoyance to Thais, then to be fair they should also arrest Thais who are an annoyance to foreigners. 
 Paul