No one has to subscribe to have a tree on the site.

One does have to subscribe to see public trees on the Ancestry.com sites, but not (at present) to see the same trees on mundia.com.

The subscription is for access to databases (though there are many free-access ones).

Where is the unfairness?

Anyone who puts a public tree on the site agrees to the Terms of Service, which basically say that Ancestry.com can redistribute user-entered data wherever and to whomever it pleases. This is permission for Ancestry.com to do this. One is always well-advised to understand Terms of Service ~before~ engaging in a site's activities.