Why can the root name server operators just drop invalid queries?
It is impossible to decide clearly what valid queries are. Making assumptions leads down a very slippery slope ending in preferring queries from your friends. The root name server operators do not want to come near this slippery slope. Dropping queries at the server would not solve the problem of the network load these queries cause. Finally it turns out to be easier just to answer all queries than to spend resources trying to decide which ones to drop.