I doubt most cloud providers are even technically ready for true prepaid billing (which requires things such as estimating and reserving funds prior to paid operations, corresponding real-time two-way interfaces instead of just eventually consistent billing event aggregation etc).

In early mobile networks, the feature set for prepaid used to always lag behind, since real-time billing wasn't really a design consideration from the beginning.

I suppose rather than taking on that extra work or offering a reduced feature set or by building something best-effort and taking financial responsibility for its failures, if cloud providers can just get away with making this the user's problem, why wouldn't they?