You wouldn't have the 1,000 paying customers without the 1,000,000 eyeballs to get them with then. It's not like it costs Weebly much to run those websites, and they essentially act as free SEO for their pay service.
Small costs can quickly balloon when you reach figures of a million plus.
The SEO is certainly a plus, however this exposure still needs to turn into dollars in the bank if you want to keep the lights on.