its not nefarious - most likely the patient statement printed and sent before someone corrected the coordination of benefits (i.e. right insurance, right copay etc), usually triggered by you.
So you got a paper that was outdated, that's it.
Its actually typical for a patient to have a huge balance, then get sent the amount, so the patient has ownership of the problem. That usually results in patients calling and correcting whatever error they (or staff) made that made balances ballon.
Unfortunately, as long as everything including a paper cut is supposed to be covered by a regulated health plan, that's what we are going to get.
All you can eat tends to increase the cost to provide it, if someone else is paying for it.