What incentivizes non-miner nodes to propagate transactions?
The incentive to propagate a transaction is so that adjacent peers cannot distinguish what transactions actually belong to the origin node. 

Since all nodes by default relay all fee-paying transaction, it is not possible for a malicious actor to determine which transaction actually belongs to you as transaction that contain your inputs/outputs of interest will be masqueraded among transactions of interest to anonymous peers. This implicitly grants you more privacy since it makes it harder for a malicious observer to correlate transactions to your peer identity (e.g. ip address).