Once they allow one sinner to go unpunished, they're no longer on sound legal ground. The courts may find it justified to order someone to stop doing something if the offense is serious and the offender is hard to find, but if a 10-second Google search turns it up, they'd not be so charitable.

The court will essentially believe they've neglected to maintain their copyright. That's a burden on copyright holders that often goes unattended.

Once you publish, you have to perform due diligence in an effort to defend your copyright. If you fail to do that, the legal strength of your rights may be attenuated. It's like a property easement. If you allow your neighbor to walk across a part of your land in order to get to his property, or to do something else, for a time, and then later decide you don't want him to do that any longer, you may find that he has the right to have access via your property. Your property right not really be what you'd imagine, but ...

RE