It's abuse because you're forcing others to download/store your data against their free choice.
Funny to see this line of reasoning from the guy who defended putting religious messages in the blockchain.
Every full node must download the full blockchain (prunable or not!).
Every full node has consented to download and store financial transactions.
NOT every full node has consented to store anything else.
You need 100% consensus for this, not merely some subset (ie, not miners; not developers) or even a majority.
Furthermore, everyone is free to store data that isn't in the blockchain.
There is nothing to be gained by putting it in the blockchain except that you force it on those who don't want it.
Explain how this is anything but abuse...
The fact is, arbitrary data can be stored in the blockchain by anybody at any time. It has been and is being used for this purpose already. Everybody running a Bitcoin node should already know this, and if they aren't, it should be a part of the notice that comes up when they install Bitcoin-QT (if there is one; I don't recall seeing one). Any Bitcoin transaction could just be a simple movement of funds, or it could be a love note, or it could be a trigger for setting off a bomb.
Eliminating this possibility would kill Bitcoin, period.
Maybe there are some ways of doing this that are more harmful than others. Maybe you can eliminate the most harmful ones, maybe not (it seems that using unspendable addresses to store data would be the "last resort" which could NEVER be eliminated from Bitcoin, and that this is one of the options that is most harmful).
It seems that the Counterparty devs are being very reasonable here. My instinct would be to say "screw it" and just continue as planned, and if future changes to Bitcoin require changes to Counterparty, then so be it.
Furthermore, for some to come in here whining "why haven't they reached out to the Bitcoin community?" "Why did they merely flip a switch without consulting anybody?" I merely present this thread, which has been openly discussing Counterparty and its "built on Bitcoin" design for months now, here on the home forum of Bitcoin, with over 6000 replies. If that's not "reaching out to the Bitcoin community" then I don't know what is.