Honestly I have only once had a f2p acc banned, and it was 10 days into its 14 day trial. I've botted and goldfarmed my fair share of f2p accounts all on a flagged ip and only had that one ban. Maybe they hope that if they let us bot f2p we might say "fuck it I'm not getting banned" , spend the cash to get mems and then they ban us. On those f2p accounts, the 2 I made members were banned within 1-2 days after.