I really have zero issue with what Damian Lillard did. In fact, watching the video clip, the response of the announcers and opposition is almost laughable to me. It... was a dunk. He didn't scream, pound his chest, flip off the opposition, kick Joakim Noah in the balls. It was just a random two points at the end of a decided game.
To be clear, I do believe there are instances where it may not be deemed acceptable. For instance, in high school there are often times where the level of one team can vary greatly from the level of another. They're all technically "high school", but if Oak Hill Academy played my former club, it wouldn't really be fair. And in that case, if Oak Hill's winning by 84 and they placed a starter back in the game with 30 seconds left so he could sit at one end of the floor and attempt a 360 between-the-legs flush, I think I'd be a little salty.
Other than that though, to use the high school example again (and oftentimes college), both teams usually play until the final buzzer. I've played for and watched teams that got dunked on during garbage time. I didn't get pissed about a supposed lack of sportsmanship. I was just bummed we were getting beat and just got dunked on too.
What ever happened to playing until the final buzzer in the NBA? When and why was it deemed so unacceptable for a millionaire (playing in front of tens of thousands of fans who paid his salary so they could watch him do neat things) to pull off a random dunk at the end of the game and then quietly run up the floor? He clearly meant no ill will, yet the announcers reacted as if he committed a felony.