I'm guessing it could be implemented by changing the way FAP works. If your usage is less than the FAP level they could allow a switch to temporarily switch FAP on, then allow you to switch it back off only if the usage isn't exceeded.
The problem with this is the expense and trouble to implement such a thing vastly outweighs its utility, especially for the majority of the customers' technical ability and understanding. You have a ton of people on here complaining that there's not enough data (for whatever reason) and don't know how to use the VDS as it is. Why complicate things even further with yet another option that no one else would understand how to use and is just as difficult for admins to explain?
Bottom line is really that HN provides you a pipe and provides tools and resources to adequately manage it. How you choose to use that pipe is entirely up to you.
"Just because you can do a thing doesn't mean you should."