At the end of the day Bitcoin is a payment system not a cloud storage. So whether they use the witness exploit to inject arbitrary data to the chain or OP_RETURN, it can be categorized as abuse and when it is done on a large scale we can call it spam.

However, OP_RETURN is a tolerated and standardized abuse that bitcoiners came up with to prevent abusers from using output scripts to "inject data" into the chain and create unspendable UTXOs.