In some cases, the person wasn't licensed when they moved into the HOA & didn't realize the restrictions.
In some cases, the person didn't read the fine print, or it wasn't disclosed.
In some cases, the restrictions didn't exist or weren't as severe at the time the home was purchased, but were imposed on the homeowners by the HOA after they took possesion. Or the rules weren't being strictly enforced then, but are now. Or the HOA has made the new rules retroactive and won't recognize grandfathering.
And in most cases, the person didn't think the rules applied to them. Or didn't pay any attention to them in the first place.