Unity doesn't want to have be the "cops" and "prosecutors" who have to go around to find, investigate, adjudicate and enforce violations in the store agreements. They want partners who, having agreed to the store terms, will make reasonable efforts to adhere to them.
E.g., Unity may have come to the conclusion that VLC violated the terms of the store agreement not by simple mistake or a reasonable different interpretation of the agreement, but on-purpose or through negligence. In that case, the reasonable thing to do is to simply stop doing business with VLC (if it's not too costly to do so), because of the relatively high likelihood it will happen again.
Don't forget that Unity could be sued for violating the terms of LGPL'd software. If they "have received a report" of an LGPL violation, they have to take it seriously. Any future lawsuit would likely be more successful if they had previously received a report of a violation and did not resolve it..
