"Free" in this case is a relative term, though. Is the time it takes to write the ad and (in some cases) code it free? Is the time it takes to field responses free? And, if the ad is placed somewhere in which most of the responses have a very low propensity to actually buy your services (or if it's simply a really bad ad that attracts the wrong audience), how much time have you squandered away? If your time is worthless, then yes, CL (or any other PMB) is free. But I imagine that most of the providers posting ads wouldn't think of it that way.
Which is why, if you watch CL closely, some advertisers have made something of a science in the way they put together their ads. Paying attention to this can make us better consumers. For providers, it can save them a lot of time (and make them a lot of money).