Small market teams don't really have a choice but to give their stars these contracts. He and CJ put butts in seats. A roster of young kids that can't win a game doesn't put anyone in a seat. That's bad for the team and bad for the league. He is much more of an investment than just a point producer.
That being said, he is doing portland a favor signing it this season. He could sign it next year and that makes him more of a liability. Everyone knows that portland has peaked, but with LAC and LAL not quite ready to compete, Denver not quite there, and OKC/SAS not able to put together a solid roster that can win right now, maybe Portland is 1 GSW injury away (and a healthy roster for themselves) from stealing a series and getting swept in the finals. Steph or Klay go down in that series and Portland is in the finals. Everyone would get a max deal that could and it would all be dumb contracts but it would also have been a "thank you" investment.
We all knew that no one in the west is competing with GSW this year, and maybe not next year either given how they have played without durant. So Portland has a 2 year window (this and next) to play at their highest level (conference championship appearance) and to create a local legend in Lillard. They have to do it. If not, they lose CJ and Dame and then what? They sell 50% of their tickets or worse for the next 10 years? Until another Greg Oden comes along and gets them 2 half seasons out of 5 where attendance is at 75%?
Pretty much have to pay the guy, just like OKC had to give the deal to Russ.