They are parasites in this way: Rather than them spend the time and money to upgrade the infrastructure they use that relies on "features" or "quirks" of IE6, they choose not to expend those resources. This just causes everyone else to spend that time and money to support their broken browser. The time and money spent by others to keep their websites usable by this trainwreck of a browser would have been better spent elsewhere. They are making others waste resources for their benefit. Pretty much the definition of a parasite.
Where I work, we spend 4-5 days developing the primary website. We have to avoid doing anything "fancy" or "efficient" with our coding, keeping it as plain jane as we can. Then we have to spend another day or so "fixing" the perfectly reasonable code, css, and graphics to work with IE 6. That's a couple of days we are not spending on the next site.
I have seriously been considering some kind of redirection (at worst) or some other kind of gentle reminder to encourage IE6 users to upgrade. Remember that if you are not using IE6 you are likely a few years behind on patching your windows system overall.
I am personally of the opinion that coddling IE6 users in this fashion gives them no incentive to upgrade - there is no pressure at all. This is holding the internet back, frankly, and at some point I'm going to just start redirecting IE6 users to an appropriate page where they can upgrade to IE7, IE8, or better yet, a standards compliant browser like Firefox, Chrome, Safari, or Opera. Stuff that was written to require ActiveX controls was broken to start with.