I don't think its unreasonable for a DM to expect control over what sources they allow into their DnDB campaign, it doesn't seem like a crazy feature to expect and it makes it a little easier for the DM, buuut considering the pushback I must be the odd one out.
How much control should a DM exert over player options for games they run? Different DMs are comfortable with different degrees of control, of course. From highly restrictive to anything goes!
But how do you exercise the level of control of character creation you are comfortable with? For me, the easiest way is to let my players know what options I'd like them to use, and then trust them to stick within those boundaries. I trust my players.
Sure, every once in a while I have a player who "cheats" in some way . . . and if I even notice it, I reflect on how much it is impacting the game and everybody's fun, and then address it with that player. If the player's cheating is having a significant impact and I've learned I cannot trust them . . . they are no longer invited to my game. This has happened only rarely, and has never gotten to the point where I felt I needed to boot someone.
But I didn't really start DMing in earnest until college, so perhaps I've been blessed with mature players.
I do run an afterschool game for middle-schoolers . . . and yeah, some of them cheat. On their homework and on their characters! It's usually pretty obvious, and we have a chat about playing fair. Still haven't had to boot anybody.
Still, it would be nice if D&D Beyond had an option on the DM's side to restrict sources and options. Not because I don't trust my players, but it would make character creation easier for them if they can only see the options we agreed upon on Session Zero. But, DDB has a long list of feature updates they are working on, so I'll be patient. It's not high on my priority list.