You’re right, Erik Johnson played for the Gophers for a year.
But there is a difference between the baseball and hockey situations. In hockey, basically anyone in college can be drafted in any year. Hockey players can (and occasionally do - see Kyle Okposo for a relatively recent example) also turn pro in the middle of the season as well. I’d rather basketball follow the baseball direction.
An interesting example is Chris Kreider. He played on the Boston College NCAA championship team this year and is now playing for the Rangers in the Stanley Cup playoffs. Nobody seems to find that scandalous. Who got hurt? (Answer: the guy who’s roster spot he took.)
The difference is that he left after the championship game. I don’t have as many problems with that, and it happens on a regular basis. Okposo, on the other hand, joined the Islanders in December after playing the first two months of the season for the Gophers.