The only price that can be paid for copyright infringement is a set of damages.
Note that there are generally two types of damages - statutory damages deal with how much your actions cost me, while punitive damages are a way of saying "no matter how much actual damage you did to me, the judge wants to discourage both you and anyone else from acting like this in the future". The two are not mutually exclusive, nor are they linked.
So if ABC is ruled to be doing zero damage, there is no price to pay for the infringement and it can continue in perpetuity as it is doing no damage.
So, be careful here about the "ruled to be doing zero damage". ABC may not have caused any statutory damage, but there may still be punitive damage. And even if ABC was found to not have caused any damage in this instance, it in no way says they can continue in perpetuity. Or at least, there's no guarantee of such. It really depends on the reason for the lack of damages (ie, if it was because the usage was in line with fair use, then fair use can continue; but if, for example, you discovered ABC was infringing on your copyright before they actually published the supplement, you can demand they stop, the courts might not award statutory damages (since ABC didn't release the product), punitive damages may or may not be assessed (likely depending on how nice ABC's lawyers play with the court), but that does not mean they can now continue to publish their product.
Long story short: no damages does not automatically mean continue on for perpetuity.
Now, web site "A" who is offering a free service can be demosntrated to be doing damage. Every time somebody uses the service, that's a DDI subscription of a Power Cards sale that WotC does not realize because the end product was delivered by web site A.
So if it can be demonstrated from web logs that web site "A" had 9000 individual PDFs created, WotC wouldl be within their rights to demand $45,000 in damages (9,000 times $5/month DDI subscription).
Failure to enforce the copyright immediately would decrease the value of the copyright by $45,000.
The damages might be reduced, but the value of copyright isn't... copyright has no explicit monetary value, absent of licensing revenue or goodwill capital.
Scenario:
Site A reproduces WotC material starting in January. WotC discovers it in March. If WotC respond immediately (where immediately is defined by the courts), Site A is liable for statutory damages for two months. If WotC decides to wait for X amount of time (say, a year), Site A can probably get that difference of time excluded from the statutory damages; they're still liable for the first two months, but their (competent?) lawyer can probably argue that WotC delayed responding to run up the damages.
In either event though, once WotC responds, assuming the material is found to be infringing, the site is not allowed to keep it up.
More so, the copyright on WotC's material is not worth any less. Statutory damages may be lessened, but litigation income is not tied to the value of copyrighted material. And any delay in responding to this case has no bearing on any other infringement case that may be pending.