They could put adverts on the f2p game, just like they did many years ago 
> Mid kill trying to slay Zulrah
> Advert for ED pops up
>
DiesTbh though, I would imagine that it's not free because it could increase the number of bots? I play another mobile game and every minute of every day there's a script bot shouting that there's a "limited deal" and that you should "act fast", which takes you to a phising site.
Hurry up, RS3 Mobile