You're right, spam is not exactly the right term here, what is happening here is that Coinbase maliciously refusing to optimize their transactions in order to either convince community that Bitcoin needs bigger blocks or that altcoins like Bcash and Ethereum are superior to Bitcoin because they have cheaper fees.
You can still argue that it can be considered spam since there's a solution they refuse to implement.
And unlike with traditional spam attacks, Coinbase is using funds of their customers to fuel this attack, because most of the time it's them who are paying the fees.
And that's exactly why we need more competition when it comes to these simple UI exchanges. Coinbase clearly (intentional or not) are unable to cope with increasing fees and they will have to radically change the way they process their transactions.