ACR LGT VIOLATED CIRCLING MINS DOING A SIDE STEP FROM RWY 21L TO RWY 21C AT DTW.

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I FLY FOR A MAJOR AIRLINE THAT HAS ARTIFICIALLY RESTR IT' CIRCLING MINS TO 1000 FT; 3 MI; REGARDLESS OF WHAT THE APCH PLATE SAYS FOR MINS. I THINK THIS IS A GOOD POLICY! 1000 FT AND 3 MI IS DEFINITELY A REALISTIC MIN FOR CIRCLING WITH LARGE TURBOJET TRANSPORTS. ON THE DAY OF THIS OCCURRENCE; WE WERE ON AN ILS TO 21L AT DTW. ATIS WAS GIVING 900 OVCST AND 4 MI VISIBILITY. THE TWR TOLD US THAT WHEN WE BROKE OUT WE COULD PLAN ON 21C. WITHOUT THINKING OF ANY CIRCLING MINS; WE SAID OK. WE BROKE OUT AND SAW THE RWYS ABOUT 1700 FT MSL (ABOUT A 1000 FT AGL); AND TOLD THE TWR WE HAD THE RWYS IN SIGHT. HE SAID WE WERE CLRED TO LAND ON 21C; SO WE STARTED OVER. AS WE WERE ROLLING OUT ON FINAL FOR 21C; THE TWR CLARIFIED OUR CLRNC; TELLING US THAT WE WERE CLRED FOR THE CIRCLING APCH; SINCE THE FIELD WAS IFR. I GLANCED AT THE ATIS; SAW THE 900 FT; AND KNEW AT THAT POINT THAT WE WERE TECHNICALLY NOT ABLE TO ACCEPT A CIRCLE; BUT BY THAT POINT WE WERE ALREADY ON FINAL FOR 21C AND TO GO BACK TO 21L WOULD BE UNSAFE; SINCE WE WERE ALREADY STABILIZED AT ABOUT 500 FT AGL. WE LANDED ON 21C WITH NO PROBLEM. I FEEL WE WERE REALLY LEGAL SINCE WE BROKE OUT AT 1000 FT AGL AND HAD 4 MI VISIBILITY; BUT I SUPPOSE WE WERE TECHNICALLY NOT LEGAL ON THAT MANEUVER SINCE ATIS WAS STILL CALLING 900 FT. HAD WE THOUGHT OF THE MANEUVER AS A 'CIRCLE' INSTEAD OF A 'VFR SIDE-STEP'; WE WOULD HAVE SEEN THE 900 FT AND REFUSED IT. AS IT WAS; I FEEL THAT ATC'S NONUSE OF THE TERM 'CIRCLE' UNTIL AFTER WE HAD ALREADY SIDE-STEPPED TO 21C CONTRIBUTED TO THIS PROBLEM. I THINK THAT UNLESS THE FIELD IS VFR; ATC SHOULD USE THE TERM 'CIRCLING APCH' INSTEAD OF SIMPLY OFFERING ANOTHER RWY 'WHEN YOU BREAK OUT.'