Wikis cost money, right? Somebody's got to pay for all that maintenance, disk space, bandwidth, etc.
And who better to pay for it than those who use it?
And what better way to decide who pays how much than by measuring how much they use it?
This pattern makes perfect sense, if you ignore everything meaningful about what makes a wiki worth paying for in the first place. Since charging users proportionally to their use effectively punishes them for using it, it deters usage, braking and even reversing the virtuous cycle known generically as the network effect, in which value attracts users, who create value, in turn attracting more users.
Truly enough, if the value could be quantified, each expenditure of usage would be seen as a low-risk, high-gain investment, which would encourage use. But since it can't, there is no quicker way to kill a wiki than to announce that you will charge for usage.
These are actually examples from other fields that show the network-effect-killing results of pay-as-you-go pricing:
The problem in both of these cases was that every time a customer contemplated using the service, a calculation had to be made: will this make my monthly bill higher than I want it to be? Even though the answer was usually “no”, it put a damper on their usage.