The celebrities are very influential and lots of youths idolize them, and encouraging gambling would be conveying the wrong message. Although it may be legal, it may still promote unsafe behaviors, particularly to teens that are most inclined to risk taking having little awareness to the consequences. They should also do something positive on their platform and not on something that may end up affecting their audience negatively over time.
A bit off topic though, but I do find sense in your post. I think the fact that most of these celebrities pages are not censored for different age groups then they should be conscious of what they are promoting whether legal or illegal, the fact that teens who are not yet up to the legal age of gambling are constantly viewing their contents makes it a bit more sensitive for them to choose carefully what platform they should or shouldn’t promote. If their contents were rated +18 in the first place then I’d understand why they’d be supporting platforms like this but for a celebrity who is a musician and sings for both children and adults to be promoting something like this isn’t really a good thing.