I think it depends on where the bottleneck is, and what they're actually trying to prevent, as to whether or not rate-limiting would actually help anything.
If they are blocking source IP addresses explicitly, it could be for a more specific concern we're not aware of, like trying to limit the amount of email "spam" that would be sent out from automated requests. A rate-limit wouldn't fix that issue, only slow it down.
We're all also assuming any of this is even intentional on their or anyone's part.
Well it wouldn't be news if it wasn't newsworthy :p