A B747 AND A B757 ARE SET UP ON A VISUAL SIDE-BY APCH TO SFO RWY'S 28; TOLD TO MAINTAIN VISUAL SEPARATION; CLRED FOR THE APCH AND CLRED TO TWR FREQ. THE B747 STARTS APPROX 300 FT BEHIND THE B757; BUT COMES ALONG SIDE-BY AT APPROX 500 FT. TWR LCL CTLR OBSERVES SIT AS B757 FURTHER REDUCES PWR FOR LNDG. TWR ADVISES B747 TO REMAIN BEHIND B757 AND IS TOLD UNABLE. TWR SENDS B757 AROUND. B747 PIC STATES APCH CTL SHOULD HAVE MADE ADJUSTMENTS FURTHER OUT AND PLANNED FOR THE B747 FLC TO REMAIN BEHIND THE B757 IN ORDER TO PLAN FOR THE SHORT FINAL; REDUCED SPD OF THE B757.

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APCH CTL (BAY APCH) ASKED US (FLT FROM HKG TO SFO) IF WE HAD A B757 IN SIGHT LAND ON RWY 28R. WE REPLIED THAT WE DID. APCH THEN SAID; 'THE B757 HAS YOU IN SIGHT MAINTAIN VISUAL SEPARATION CLRED FOR THE VISUAL TO RWY 28L.' THE 4 PLTS IN THE COCKPIT DISCUSSED THE CLRNC; IN THAT WAS THERE A RESTR NOT TO PASS THE B757. WE CONCLUDED SINCE THERE WAS NO SUCH STATEMENT THAT IT WAS NOT PART OF CLRNC AS LONG AS WE COULD MAINTAIN VISUAL. THE FO FLEW THE APCH APPROX 100 YDS BEHIND THE B757 AT MINIMUM SPD OF 149 KTS UNTIL 500 FT AGL WHEN THE B757 SLOWED DRAMATICALLY. TWR THEN CALLED AND SAID 'NO NOT PASS THE B757.' THIS WAS THE FIRST TIME WE HEARD THAT RESTR. AT THAT POINT WE WERE ABEAM THE B757 AND COULD NOT SLOW FURTHER. I TOLD THE TWR THAT 'WE WERE UNABLE TO COMPLY.' THE B757 WAS ISSUED A GAR CLRNC (WAKE TURB ISSUES). NOTE: IF NOT PASSING AN ACFT IS SO CRITICAL THEN I FEEL IT SHOULD BE STATED IN THE CLRNC; AND NOT ASSUMED. I CALLED THE TWR SUPVR TO SUGGEST THAT APCH MAKE IT CLR WHAT IS REQUIRED WHEN 2 ACFT ARE ON A VISUAL APCH TO RWYS 28.