19.5% 3 point shooting percentage for the season.
No one seems to want to talk about that. Ruins a good story I guess.
Probably because it's up 10% since his first 4 games. Really speaks to how lofty his expectations are that, at 21 years old and averaging 26/9/7 on a .500 team missing it's second best player, people will bring up a cold spell from 3.
He's been a below average 3 point shooter since he came into the league and he usually takes 7 to 9 attempts per game. The percentage rate hasn't really justified the volume to this point in his career. That's absolutely a fair criticism for Luka.
Thing is, he's taking tons of step back 3s. These are possibly the most difficult shots to master in basketball. At the same time, they are unguardable! It's not like Luka is a spot up shooter who's taking uncontested shots. Very few players around the NBA are crafty enough to have the step back 3 in their arsenal. Thankfully, we got one of them in Tatum! 
They're not unguardable if he's only a below average three point shooter, though. On the other hand, Tatum is a career 40% 3pt shooter who's currently shooting 43% from 3 this season. He still gets crapped on for "hero ball" when he takes a step back 3 during a game. Imagine this board if he only shot 32% for his career and started out the season at 20% but still launched 8 threes a game? We'd be beside ourselves about his shot selection, no?
No one is doubting that Luka is an otherworldly talent. But he's not above criticism. That's a part of his game he needs to improve.
Step back 3s are unguardable in the sense that opposing defenders cannot contest them. All they can do is hope that Luka misses his shot.
Tatum (...) gets crapped on for "hero ball" when he takes a step back 3 during a game
If Tatum were averaging 7.3 assists per game for his career while keeping literally everyone involved on offense, I bet nobody would ever accuse him of playing hero ball. 
If Luka is going to hit them at a below average rate - which he has - that's a shot you live with if you're an opposing defense and I think that's how some teams are starting to play it. Stephen Silas had an interesting comment on it awhile back, when he was an assistant in Dallas, they tried to discourage Luka from taking step back 3's because it was an inefficient shot for him. But they realized that opposing teams were respecting the shot and selling out to defend it, so they let it go. Now, if you're a team scheming against Luka, you allow your defender to give him a little space to protect against a drive into the paint where he's been better. To start this season especially, he's been more effective in the midrange and since he's been out shape, he's playing a bit of bully ball in the post which has also worked for him. For all of the posts about how great and historic Dallas' offense has been in the past, they haven't been that team to start this season (and before we get to Porzingis, he missed time last season too and Dallas was able to maintain offensive potency).
And all things being equal with Tatum, I maintain what I said if roles were reversed. Posters would find other things to rag on. "Why doesn't he play defense?" "Why is he so out of shape?"