ON APCH TO BOS RWY 4R AT 1200 FT; A B737-800 FLT CREW RECEIVE A TCASII ALERT DUE TO HELI PASSING FLT PATH AHEAD AT SAME ALT.

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APCHING BOS RWY 4R NIGHT VMC ON THE VISUAL APCH. JUST PRIOR TO XING OVER THE HARBOR ON FINAL APCH; THE TCASII MOMENTARILY ALERTED TA AND THEN 1 SECOND LATER TO AN RA FOR (WHAT TURNED OUT TO BE) A POLICE HELI CLBING L TO R; XING OUR BOW CO-ALT ROUGHLY 1 MI IN FRONT OF OUR ACFT. AS THE 'CLB' COMMAND WAS BEING BLARED INTO OUR COCKPIT; BOS TWR RPTED THE HELI AS TFC (FOR THE FIRST TIME) AND MY COPLT LATER REMEMBERED HEARING IT MENTIONED THAT THE HELI WAS SAID ALSO TO HAVE RPTED OUR ACFT IN SIGHT. AS THE HELI MOVED TO OUR 1 O'CLOCK POS AND NO LONGER A FACTOR WE ACQUIRED IT VISUALLY AND DISCONTINUED THE CLB MANEUVER COMMANDED BY TCASII AND SALVAGED THE APCH. A SUBSEQUENT CONVERSATION WITH THE TWR SUPVR ELICITED AN APOLOGY ON BEHALF OF HIS SUBORDINATE FOR NOT RPTING THE TFC TO US PRIOR TO TCASII DOING SO. STILL; I HAVE CONCERNS ABOUT THE POLICY IN PLACE. REGARDLESS OF WHETHER THE HELI THINKS HE HAS US IN SIGHT (APCHS WERE ALSO BEING CONDUCTED TO THE PARALLEL RWY) I STRONGLY OBJECT TO THE MARGIN OF SAFETY BEING ERODED BY HIS BEING AT OUR ALT AS HE CROSSED OUR EXTENDED RWY CTRLINE. HAD HE REMAINED WELL BELOW THE GS UNTIL WELL CLR OF THE APCH CORRIDOR; THERE WOULD HAVE BEEN NO PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED THE HELI PASSED AHEAD APPROX 1 MI AT HIS ALT. THE RPTR WAS NOT SURE ABOUT THE TIMELINESS OF THE TA; AS THE FO LATER ACKNOWLEDGED THAT HE HEARD THE LCL CTLR ADVISE THAT THE HELI HAD THEM IN SIGHT. RPTR'S MAIN CONCERN IS THAT ATC SHOULD NOT ALLOW TFC TO CROSS IN FRONT WHEN ON AN ILS APCH INSIDE THE OM; BUT SHOULD BE FORCED UNDER AND BELOW UNTIL TFC PASSES.