1) But they supported this before on GAE. GAE had 'spending limits'.
2) Also if they are able to figure out when you've hit your daily free quota and cut you off almost immediately, how are they not able to figure this out?
If I recall correctly, GAE is an example of something they made specifically to be a cloud product. Products like Compute Engine, GCS, Bigtable, and Pub/Sub are things developed internally and then sold publicly once they realized others might find them useful. Perhaps the products developed first for internal use weren't developed with features like measuring billing usage in real time in mind.
2) Also if they are able to figure out when you've hit your daily free quota and cut you off almost immediately, how are they not able to figure this out?