TRACON Controller described a TCAS RA event after traffic separated by VFR requirements and having one another in sight took evasive action; the Controller questioning TCAS responses when involved traffic has one another in sight.

Narrative

A BE1900 was enroute. A helicopter was crossing northeast bound (right to left) at 105; VFR. Traffic was issued multiple times to the BE1900. The helicopter was being worked by an adjacent Approach Control. The Developmental issued a 20 degree turn to the BE1900 to avoid the aircraft crossing and the potential for a TCAS event. The BE1900 could not hear the Controller instructions over the TCAS Alert in the cockpit. The BE1900 replied that they were responding to a TCAS RA and were climbing. The BE1900 climbed to approximately 115 before resuming ATC instructions. While TCAS is an extremely useful tool; the air carriers have to change their regulations whereby a pilot must comply with an RA; especially in instances where visual separation is being applied or when a Controller has aircraft properly separated. This is an almost daily occurrence in our area. Further; TCAS equipment should never be so loud as to drown out a Controller's instructions. This could have been a much worse event if circumstances were different.