Having been through similar decision making processes myself (with the Google Custom Search API, the Google Translate API, etc), this is just as likely an abuse mitigation technique as it is a revenue generation opportunity.

Requiring even a modest at-cost fee for a web service does wonders to discourage all sorts of misuse, from wanton large-scale data mining, to blatant repacking and resale, to worse. (Heck, simply requiring a valid credit card alone helps.)

And sadly no, simply having low quotas for free access doesn't entirely suffice. If there's material value to be extracted from a free service, you'd be amazed at the lengths people will go through to create large numbers of low-volume scrapers. Most of these are obvious and easy to detect and defeat, but continually doing so adds up in cost, and it takes engineers away from providing better services to legitimate customers.

In short, most people on the outside don't appreciate just how difficult the handful of bad guys make it for companies to do something good for the other 99%. So I'm sympathetic to Microsoft here, I really am.