The (subjective) sin in question is gambling but I think poker is unfairly lumped in with games of chance (blackjack, roulette, slot machines etc.), probably because it takes place in a casino.
However it is a statistical game of incomplete information. Just so happens to be played with cards. I don't see any relationship with gambling exepct for the bet sizing but this is more like choosing what to invest in relation to your expected return.
If playing an MTT you are not even risking cash on every hand and your inital investment is capped at the buy-in.
Unfortunately, the vast majority of people do not recognise this and the stigma is hard to overcome.