I'm afraid that a large amount of bots doesn't actually come from free-trial accounts, but regular players that simply don't feel like playing along with the classic reward baiting design of MMOs. Instead of forcing themselves to the boring grind to get a reward, they use bots to reap the rewards while effectively skipping the grind.

Hence why SE is rather hesitant to just ban them. They are paying customers and they don't want to lose the revenue. The free trial is already extremely limited as is.