You're right - it *should* be metered, but the real issue here is that many ISPs advertise that they offer 'XXmbit per second' & do *not* mention a cap or any kind of actual tiered pricing or metering. Then, when you "over consume" (meaning use more than they think you should, more than some magic [invisible] number that they have come up with), they get upset.
I'm all for metered pricing - but only if it's actually what I'm buying. If I'm buying based on a XXmbit per second price structure & no metered structure (or cap) is advertised then I expect that I can use as much total bandwidth as I would like. The problem is that, as of late, no matter how a plan is advertised (and *very* few are advertised as metered, at least at a consumer level) they're usually imposing some sort of cap or metering system, even if that is *not* the way the product was originally marketed.