I see that the limit you mention is for dynamic maps. Somewhere else, they say "Requests for Dynamic Maps and Dynamic Street View (Street View Panoramas) will be billed at a higher rate than requests for Static Maps and Static Street View. Dynamic Maps and Street View images respond to user input; the Static versions do not."

I imagine that this is an incentive for developers to use static maps (which you could replace with dynamic ones when the user clicks or hovers on them). If you used static maps alone, that's 300K loads per month, or $480. You would deduct the $200 credit, for a $280 bill, but then pay full price for all the other services. I can see how that adds up quickly...