Why get in the way of relieving people of their money in advance?
An initial search found this interesting thread going back a over a decade.
Ed: I like the follow-ups... being from the future, this just reminds me how little we matter to companies like Amazon even back in 2010 when we communicate through their forums.
There's more here once I refined the search further.
Looks like a combo of people who have to pay per transaction fee(s) and people where its easier for them to authorize $1K for cloud services in one go versus $80 one month, $120 the next, $90 the following and so on and so forth but don't want to commit to their annual offerings.
It's the best of both worlds for Amazon, per second rates AND money up front.