The argument as I understand it is that copyright is a property right (agreed) and that so long as it exists, someone owns that property. I can see some merit in that argument. Following it through to its logical conclusion copyright exists until its statutory term expires at which time it ceases to be property and cannot be owned.
At common law, if you abandon property then the property belongs to the first person to claim it. However, there is some doubt in my mind if the quote you use is a statement of abandonment or instead an irrevocable and unconditional licence granted to everybody. It seems clear that the intention is the latter and not the former, particularly when the document is considered as a whole; that is, it is not the expectation of the person making it that the next person to come along can assert ownership of the abandoned property.
I do not agree with the arguments advanced that an irrevocable licence can be revoked because there is no contract in the absence of consideration. On that basis, the recipient of any gift could never rest comfortably on its title if the giver could return at any point in the future and reassert their ownership. Once a gift is given then title has passed; even if the title is only to a licence rather than the copyright itself. To be legally effective, the giver must have the intention to make the gift (pretty unassailable here), must deliver the gift to the receiver (posting it on a web site would do that) and the receiver must accept it - technically the gift could be revoked up until the point where any of the beneficiaries (i.e. anyone) downloaded it, after that it would be irrevocable.
As to the second part of your question, if it was illegal for the work in question to be created, distributed or possessed as it is, for example, with child abuse material, then releasing it into the "public domain" does not relieve the creator, distributor or receiver of any criminal liability they might have.
