Is WTB Tubeless Compatible System rim incompatible with clinchers?
All tubeless compatible (aka "tubeless ready" or just "tubeless") rims and tires can be used with tubes. It would be impractical even for bike industry standards to disallow people from putting in a tube when something goes wrong, or even buy a modern wheel/rim if they weren't interested in tubeless.

The main special thing about the TCS rims compared to many others is, as their ad copy points out, they have a certified UST standard bead seat. The idea there, not to give an opinion either way, is that in conjunction with a UST tire the bead interface meets a designed standard and can have greater security than the alternative of just using parts that are more arbitrarily designated as "tubeless compatible," in other words every non-UST MTB tire and rim on the market. You can still use whatever tires on TCS rims, tubeless or not.