You'd need undeniable proof that a node is a node and not merely pretending to be one. Just like we need undeniable proof that a miner mined a block. For miners we use computational work. It's non-trivial to come up with something equivalent for node running.

In general, proving something is in a digital context is very hard. It's why our emails are filled with spam, captchas get ever harder, and why SN has a cost to post.

Most people would agree full nodes are providing value to the network and should be compensated if possible.