Actually you aren't obligated... You have purchased the right to now buy tickets.
The defaults are on the PSL contract. Those ran from 10k to upwards of 50k per seat. So if you wanted 2 tickets at the 50 yard line, just the right to but them cost you 100k. In their infinite wisdom, you could finance that for a decade or more.
So that is what is defaulting. People aren't looking to continue to owe 10s of thousands to then have to shell out another several thousand just to attend a game.
I don't know how people got duped into thinking this was a good investment.