Some of these are just strange with no explanation.  Others, I think people are mistaking quotation marks for underlining, bold, or italics to emphasize words.  Even an exclamation point would work better.  There are even a blog and web sites on the topic, no joke, at http://www.unnecessaryquotes.com/ and http://www.juvalamu.com/qmarks/, among others.  Quotation marks are used ordinarily to show quotations, or attributions in fiction to a particular speaker.  They can also be used to offset story titles and other titles in part of a sentence.  An increasing use is to use them to show irony, or sarcasm.  Like you disapprove of your sister’s new boyfriend, so you write:

Yeah, he is a real “winner.”

In that case, to indicate the opposite, that he is a loser.  So why would someone post a sign:

Employees must “wash their hands.”

It is not a quote, title, or attribution, so it can only mean irony or sarcasm.  Are they supposed to wash something besides their hands?  Are they not supposed to wash?  Anyway, you get the point.  Here is a gallery of them.  “Enjoy.” [sic.]