In your example, deciding whether or not that’s a problem comes down to how you feel about people profiting from inside information. Which means maybe it’s no issue, but there are circumstances where it might be considered a problem.
Here’s a hypothetical. Let’s say you’re a player for Missouri who happens to have a close friend playing for UK. You’re also scheduled to play UK in a couple of days and Missouri is a slight underdog. During that week’s practice, Will Levis tears his ACL and won’t be able to play, and his backup is Morgan Newton. Your buddy from UK texts you right after practice with the news and you immediately pull up your phone to place a bet on Missouri before the news is public and the line moves.
As a result, you are able to pocket a bigger win because you were able to capitalize on information before it became available to the general public. Now, you personally may not see any issue with someone profiting from insider information, which is fine and you’re entitled to your opinion. I’m just pointing out a situation where others might consider a player betting on their own team to still be problematic.
Me personally, I’m not a fan of gambling period, but it’s legal now and people are free to do what they choose with their money.