I suspect this is because FirstNet customers are treated like full enterprise-class users on the AT&T platform and you can set your account up in some really accidentally stupid ways in AT&T Premier (the enterprise-level account manager system) to put obscure plans like the pay-per-use data package onto an account.

FirstNet is for all intents and purposes a large enterprise "foundation account" inside AT&T's enterprise management to which you end up a subaccount under.

What doesn't also help is that FirstNet's customer care reps are... quite frankly, frustratingly useless at times despite having to be the sole point of contact for account management. AT&T reps can't help you and send you to FN's reps.