If one is upset their work is being hijacked(used by another artist and other artist taking credit), and have taken no steps to prevent it, then they've only their self to blame. Retaining a lawyer may be a touch extreme in most cases, posting every piece if work they own on 4-5 websites is a bit careless if you're going to be offended if it's copied. Blasting your work all over the interwebs is somewhat effective, it's not the only way to spread the word.
If they have time to bitch about it, they have time to actually do something and if their silence isn't consent, what exactly is it, it's clearly not a deterrent. Anyways, the whole thing feels hypocritical in a lot if cases, for me, if you don't care enough to at least, try to protect your hard work, then don't be surprised it got jacked. Simple.