Services offer this because if you don't have any users, it's easier than building in and enforcing limits. If your service gets traction then it makes sense to spend the time and money on development, not before. I personally wasted a lot of time on building limits for a service that has yet to gain traction.

People say "unlimited rates" but what they really mean is "indefinite rates".

Because if it was static human readable text content, they'd let you push that much though just fine. It's unlikely that you ever would get to that level, but they don't mind if you do.