The cost in goodwill (and perhaps even real money) of using translate to fill hundreds/thousands of google ad-filled spam sites in hundreds of different languages may be much higher than they could ever conceivably recoup in charging for the API.
Once you charge more than any legitimate user would be willing to pay for the API, you are effectively shutting down the API anyway.
Are spammers going to be willing to pay x cents/call just to fill spam pages with translations? We must make x high enough that spammers will not make enough profit to justify the expense but legitimate businesses that depend on the API will still be able to pay for it.
Spam is all about maximum distribution and massiveness, so I don't think the pricing would have to be very high to take the profitability out of the spam operations.
The spammers would only have to translate each page once. Further, they could translate only the pages that users request in language X as needed and cache the results.
And what stops them from doing this with any of the other translation services out there? translate.google.com isn't going anywhere, and if they're actually having a problem related to abuse (as they claim on the Translate API's site), I seriously doubt that spammers are going about this in this way. The notice seems to say that the Translate API is overloaded -- charging x c/call would definitely diminish the load, especially if we consider that most spammers are not going to want to give real payment data to Google in the first place (since it can then be subpoenaed and used to prosecute or otherwise identify the spammer).
Once you charge more than any legitimate user would be willing to pay for the API, you are effectively shutting down the API anyway.