Because it gives the appearance of impropriety. Joe has enough on his plate than to give the shit heads more ammunition.
Now I'm in front of a real keyboard, let me add a couple of things.
First, I'm not sure the appearance of impropriety with Hunter is going to make much of a difference. I really doubt the obstructionists in the Senate are going to say, "Well, now that Hunter has retired off into the sunset, I think we can actually work with this President."
But also, I'm not convinced this even should be the appearance of impropriety. Forget about the fact that Hunter has a long history of being a very unlovable douche, and consider this situation:
You're starting to get old. You've had a life filled with mistakes and regret. You were never as talented and successful as your brother, and everyone knows it. Your family also endured a number of tragedies. Through all this (and perhaps because of it), you made some really bad choices, and some really weird, creepy ones, too. Finally, you're starting to settle down and get your life in order, and you've decided to change directions and find a new calling. Amazingly, you pick art, and even more amazingly, you discover you're actually good at it!
Unfortunately, just as you're getting started on this new journey, your decrepit old man, who long ago should have retired and spent his days watching
Simon & Simon reruns, decides to run for President. Even worse, he wins. You probably start history's most depressing pen pal exchange with Melania Trump. But now, because of dad, eyes are back on you, and everyone questions every dollar you make. You're not interested in government or power, anymore. You just want to paint. Just as you're heading into the twilight of your adulthood, just as you've found this new passion and skill, you're supposed to drop it all because Pops had too many Metamucils one morning and decided he should run the free world?
Long story short, as long as Hunter keeps his greasy fingers off my government, I don't care what he does. If anything, I hope he makes a crap ton of money from his art, just because it will piss people off, and that will amuse me.