You can't track individual coins, so you'd have to "taint" entire wallets. Using a mixer would taint the mixer and every wallet it sent to. I'd think this would end up tainting almost everything before too long.

Bitcoin also doesn't require the receiver to authorize a transaction, so if you had control of a tainted wallet, you could taint other wallets at will, wielding it like a weapon.

Doesn't seem feasible. Not that this always stops legislators.