Just because you paid a fee to dump your waste doesn't mean it's not waste.
Yes, they paid the transaction fee to get put into a block, but now their data sits in the blockchain and eats up storage space on every device that maintains a copy of the chain. For a financial transaction you can at least comfort yourself with the knowledge that the "pollution" is an entry in a finance ledger, but not so here. The transaction fee is set based on how much people are willing to pay right now to get a transaction through, it doesn't factor in the long-term cost to store that transaction on every machine that needs to host it in the future... that cost is ignored.
Miners decide which transactions are "pollution" and which are not. Not you, and not the parent.
Seriously, who are you to tell anyone what data is "pollution" and what data isn't? Like, do you go around to people on their laptops in cafes and tell them to delete their files off the Internet because you think they're wastes of space?
EDIT: Having a bespoke data field (OP_RETURN) in a Bitcoin transaction is an improvement on this front, because it provides a cheaper alternative to people storing the same data encoded as potentially-spendable UTXOs.
> every device that maintains a copy of the chain.
Of the full chain. You can run a Bitcoin full node in pruned mode, only using the most recent 5GB, with more or less the same security. (A 3-day chain reorg would be needed to reduce the security of your full node.)
It does make it take longer to set up a pruned node, though.
Yes, they paid the transaction fee to get put into a block, but now their data sits in the blockchain and eats up storage space on every device that maintains a copy of the chain. For a financial transaction you can at least comfort yourself with the knowledge that the "pollution" is an entry in a finance ledger, but not so here. The transaction fee is set based on how much people are willing to pay right now to get a transaction through, it doesn't factor in the long-term cost to store that transaction on every machine that needs to host it in the future... that cost is ignored.