* I wouldn't call it "spying", "spying" sounds more like with bad intentions, while the buyer can be 100% friendly, without any sinister intent.
The intentions really don't matter. If you don't treat bitcoin as fungible currency that it is, you're doing equal harm, regardless of intentions.
Why? Because any powerful instituation is already capable enough to do these analysis and has all the necessary tools at their disposal.
Treating every mixed coin as "tainted", just because you're sure some of them come from illegal activity doesn't count as a somewhat established capability. It's simply put, insane. You wouldn't treat a $10 bill as tainted if there was 50% chance it was used illegally. Every dollar bill is potentially used illegally, just as every mixed bitcoin.
You could argue that it's subjective, but some of their history in the blockchain are based on facts.
But, more than half of it is claimed to be "tainted". Every single coin which passes a mixer or is coinjoined is deemed as tainted, because a fraction of the input is involved in illegal activity. You can't seriously argue that mixed coins being reasonably called tainted is based on facts, because it is half insane and half guesswork.