Why hasn't CBS or Paramount just taken over a fan series or two?

[chardman]

Fan series get away with what they do because CBS technically turns a blind eye to their efforts. If a fan group got too much notoriety; CBS WOULD need to issue a 'Cease & Desist" order to protect their copyright on the IP. (If they do not, then it can be claimed in court that they no longer have IP rights to Star trek because they must show they are legally 'enforcing' and 'protecting' their claim, or they lose it and it slips into the public domain.)

No. This is completely wrong. A copyright need not be defended to remain in force. Trademarks, on the other hand, require enforcement to remain in force, but copyright enforcement can be as inconsistent and willy-nilly as the copyright holder chooses, without endangering those rights.

Here's a good source about Copyright myths. (See section 5) LINK