AN ACR MLG DSNDED BELOW ASSIGNED ALT.

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WHILE ON THE MILTON 7 ARR FOR LGA; BTWN PROUD INTXN AND DIALS INTXN ON THE EXPRESSWAY VISUAL; ATC CLRED US TO 2500 FT AND CLRED US FOR THE VISUAL APCH. ABOUT 2 MI S OF DIALS INTXN; HE TOLD US TO CLB TO 3000 IMMEDIATELY TO AVOID TFC. HE THEN TOLD US HE HAD ONLY CLRED US TO 3000. THE CAPT AND I CLRLY HEARD THE CLRNC FOR OUR ACFT TO DSND TO 2500. SUPPLEMENTAL INFORMATION FROM ACN 213291: THE APCH CTLR THEN CLRED OUR FLT; ACR 2; FOR THE EXPRESSWAY VISUAL; CROSS DIAL AT 2500. NEARING DIALS; THE TCASII TA INDICATED 2 O'CLOCK TFC. THE TFC WAS SIGHTED AND A SECOND TCASII ALERT SOUNDED. AT THE SAME TIME; APCH CTL POINTED OUT 2 O'CLOCK TFC AND AN URGENT CLRNC TO 3000 WITH A COMMENT ABOUT HIM PREVIOUSLY CLRING FLT; ACR 2; TO 3000. WE BOTH OBSERVED THE SINGLE ENG ACFT PASS OFF TO THE R AT A MODERATE DISTANCE. THE SINGLE ENG ACFT HAD TURNED W TOWARD US AND THEN MADE A TURN TO THE S TO AVOID OUR FLT PATH. A TELEPHONE CALL TO ATC TO DISCUSS THE TFC CONFLICT WAS ENLIGHTENING. THE CONFLICT TFC WAS AN 'ACCOMMODATION' OF THAT AIRPLANE BY ATC. THE INFERENCE WAS THAT IT WAS SOME SORT OF LAW ENFORCEMENT SURVEILLANCE. WHILE IT IS NICE TO BE ACCOMMODATING; HAVING AN ACFT IN AN ARR AREA CONDUCTING GND SURVEILLANCE DOES NOT SEEM TO MAKE VERY GOOD SENSE. IT WOULD SEEM TO ADD WORKLOAD TO THE APCH CTLR THAT HE REALLY DOESN'T NEED. FURTHER; A SURVEILLANCE OP OF THE GND MIGHT DIVERT THOSE PLT'S ATTN AWAY FROM A VERY BUSY SKY.