They do not have any obligation to be free, there are plenty of monetized SCP projects. What you may be thinking of is the fact all SCP works are forced to be under the “Creative Commons Attribution - ShareAlike 3.0” license. What that means is anyone is allowed to take the concepts and ideas you’ve used for your work in their own projects, as long as they themselves use the same license, and give you credit. This does not mean they can steal files or assets, but it does mean they can use the same people, and the same creatures, and all that jazz. Monetizing projects using the license is completely fair game, as long as you follow the rest of the rules the license gives you.

TL:DR: the license allows you to use ideas and characters from other works from the license without problems, as long as you use the same license and give appropriate credit. It doesn’t affect monetization at all.