We don't have the data to tell whether bannings in Guild Wars 2 is actually more prevalent in other games since none of the studios routinely publicize such numbers, but one can certainly imagine that removing the subscription fee removes a financial incentive NOT to ban a customer. Sure, a banned player might eventually have opted to pay for microtransactions or expansions, but it's nowhere near the guaranteed revenue of someone who is happy to pay $15/monthly for the opportunity to troll. Moreover, their conduct also affects the tone of the community in a way that might influence whether others stay and pay.
Looking beyond this issue, we are in a somewhat unprecedented scenario in which two separate major titles - Diablo III and GW 2 - launched within a six month period never intending to collect a subscription fee. Both sold seven digit numbers of copies at $60 a head. We have some free to play games that have over a million users - few of whom are likely to have paid $60. We have a small number of subscription MMO's that actually have a million former subscribers. Neither category of game intentionally chose that outcome.
Even if most players will never pay for additional transactions, both titles are franchises with at least some incentive not to ruin their respective brand names. See, for example, Blizzard's scramble to add an alternate advancement system upon determining that the base game lacked staying power, when in principle they could have shrugged, secure in the knowledge that people who run out of stuff to do have already paid. It will be interesting to see if anything more substantial than policies about banning people for abusing their group-mates in chat changes as this type of model becomes more common in online gaming.