Well, if blink-182 has been signed for 15 years, they probably didn't have a 360 deal. So the label would most likely have seen nothing from their touring profits, besides CDs sold on the road.
I imagine the deal just ran out though.
I can def see a situation where they had to either negotiate a new deal, since they were still on their contract from the hiatus, or leave the label. So they decided to just leave the label. I feel like if they really desired being unsigned, they would have discussed it more during the recent push of the last album.