I think that all boils down to, again, how complex the actions of single-celled organisms actually are. Just the fact that they do the things listed is (IMHO) cause to ponder over whether or not they are somehow sentient. If I had the means, personally, it would probably be something I would try to investigate myself. BUT, I do understand how inefficient it would be, if the scientific community took the time and resources to entertain every interesting idea. That much, I can't argue with.
By things listed, do you mean things like build homes etcetera? 'Cause personally I've never actually heard of single celled cells doing anything like that and have a suspicion that it's just something someone made up.