I think it has something to do with a random middle of the night booty call. Don't call me, I'll call you type thing? But in a guy's mind it could mean something totally different. So? Am I right?
Ever heard of the old addage "why pay for the cow when you can get the milk for free?" Well, it goes right along with that modus operandi. Basically, "no strings" means that he wants to fuck you without investing any emotional content. For some sexually liberated women, this is quite acceptable as there may exist only an interest in getting laid. However, if you are one of the women who are ill-epuipped to handle emotionally detached sex, then beware the "no strings" clause. You may wind up hurt.
At least he's making it clear from the get-go and not promising a relationship in order to slip his chip in your clam dip. In this situation you need to be brutally honest with yourself about how well prepared you are to handle any fall out stemming from sex without emotional appeal.