There's a measurable difference between the bonus and no-bonus option, and in this case, that's 5%. That is the offer. Given the option between the two, I'll take the bonus knowing fully well in either scenario they are profiting.

AT&T's classic monthly rates factor in subsidization, and thus are substantially higher than ...whatever value you seem to be wishing that they charge - I sincerely hope not at-cost. If they offer a 5% discount, they're still profiting, but you're paying 5% less than you would have. That is something, and that is an offer.

You're comparing apples and oranges. Compare the 5% bonus with simply lowering the cost of fares an equivalent amount.

The MTA is at least partly-funded by real-estate taxes.