For this case, leaving a source as Google is very lazy and unacceptable (in my opinion) at least, it shows that poster does not claim the ownership of that content.
And that should be good enough for not getting banned. I want to be able to post something I read somewhere even if I can't find the link right now. Of course when you post a large amount of text that seems to be copy-pasted, and not a paraphrased sentence, you probably should have a proper link, but it still should not lead to a ban. Is it a low value post that should be deleted - perhaps, depending on circumstances.
Same with editing posts. Moderators should not ban for something that no longer exists. Honestly I'm not a huge fan of posting plagiarism findings publicly (just report to mods) however there are circumstances where I could clearly see there was no intent to plagiarise but rather a mangled quote, or a copy of a well-known text that moderators may interpret as an attempt to claim as your own, so I would warn the poster that they should edit it to clarify the source. Let's face it, real plagiarists will get caught eventually anyway. Banning slightly careless users is not necessary.