I went for a quick bite on the weekend. After ordering, I sat down and took up my laptop to get a bit of work done while I ate. I was instantaneously told that they just could not have my laptop out once it got busy in the next hour.
I completely understand, and was not planning to stay long, but nothing makes you feel more unwelcome then having someone greet you on that basis. I think having something posted rather than feeling singled out would be more palatable. This policy apparently only applies on the weekends.