There are too many sites which do not charge. Of course, sites like Ezine articles is free, even though there is an option for paid membership.
Bottom line is:
They need us to make their money.
It is the millions of articles, hubs etc that gives them a high page ranking.
It is us, they need to make money off of the numeours google ads.
Advertisers advertise, for big bucks, on sites that have lots of traffic.
There are enough free sites, which will welcome us without charging a penny. They will laugh all the way to the bank for the new revenue we bring them from advertisers who will now pay to advertise on their site, due to all the new writers they will get.
Unless they get awful greedy, the old saying goes, "Don't bite the hand that feeds you." We feed them a lot more than the pennies they give us.
Face it, you don't want to post on any site that is not ranked high in the search engines. These sites, would lose so many writers, their rankings would drop.
A smart business person, offers Free and Paid versions. Usually, the paid versions are not worth the expense.
Besides, most have been free for years, and if it would benefit them to charge writers, they would have already done it.