While this service is pretty clear in that you just pay for each fax, one could easily imagine a free service that collected fax machine phone numbers and then later begins sending lots of fax spam.
Many years ago a company kept waking me up in the middle of the night with repeated attempts to deliver junk faxes to my home number. That inspired me to figure out how to use some fax software with my faxmodem (see, many years ago) to capture one of them. They became keen on getting me off their list when I asked for damages under the junk fax law.