Pruning nodes is centralizing nodes. We need full nodes to bootstrap other nodes. If full nodes are only run by a handful of hardcore libertarian Bitcoiners and large institutions with legal teams to fight for their ability to run full nodes, then nodes are too centralized and too easily captured.

Pruned nodes keep the past couple of days of full blocks and also relay valid transactions and blocks so it’s not as clean a solution to potential legal risks as you might assume. Tightening consensus rules to make it more difficult to embed spam within a “valid” transaction prevents nodes from relaying or storing those transactions, so it’s more protection for node runners.