MD80 LEVEL AT 12000 FT INBOUND TO LAS WERE CLRED TO THE KSINO 1 ARR WITHOUT A 'DSND VIA' AND STARTED A DSCNT AFTER LUXOR.

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WE WERE CLRED TO BETHL -- A FIX ON THE KSINO 1 ARR -- AND THEN THE KSINO 1 ARR. AT LUXOR; WE DEPARTED 12000 FT FOR THE 'AT OR ABOVE' 10000 FT ALT RESTR AT MIRAJ. SINCE THIS WAS THE FIRST TIME I HAD SEEN AN RNAV ARR; I WAS SURPRISED WHEN THE CAPT STARTED OUT OF 12000 FT ON HIS OWN. HE THEN TOLD ME THAT ONCE YOU ARE CLRED FOR AN RNAV ARR; YOU ARE ALSO CLRED TO DSND AS NECESSARY TO MEET THE PUBLISHED ALT ON THE ARR. WHEN I CALLED ZLA THAT WE WERE LEAVING 12000 FT; THEY WERE UPSET THAT WE HAD DEPARTED 12000 FT ON OUR OWN. BY THE TIME THE CONFUSION SETTLED DOWN; WE WERE AT ABOUT 11200 FT. ZLA THEN PROCEEDED TO GIVE US ALT CLRNCS FOR THE REMAINDER OF THE ARR. LAS APCH THEN GAVE US VECTORS FOR A VISUAL APCH FOR RWY 25L. WE HAD TO FOLLOW ANOTHER ACFT FOR THE VISUAL TO RWY 25L; THE CAPT DID A GREAT JOB BUILDING SPACING BTWN THE 2 ACFT SINCE APCH DID NOT LEAVE US MUCH ROOM TO WORK WITH. AFTER WE CLRED THE RWY; WE WERE TOLD TO GIVE ATC A CALL. THE CAPT CALLED; AND HE SAID THAT ATC WAS NOT GOING TO FILE A DEV AGAINST US; BUT WANTED TO KNOW WHY WE HAD DSNDED ON OUR OWN SINCE THEY WERE HAVING A LOT OF TROUBLE WITH OTHER ACFT DOING THE SAME THING. THE ROOT OF THE CONFUSION STEMS FROM THE FOLLOWING QUESTION -- WHEN CLRED FOR AN RNAV ARR; ARE YOU CLRED TO MEET THE DSCNT PROFILE ON YOUR OWN; OR DO YOU NEED AN ADDITIONAL 'CLRED TO DSND TO MEET THE PUBLISHED ALTS' TYPE OF CLRNC FROM ATC? IT SEEMS THERE IS A LOT OF CONFUSION ON THIS ISSUE BTWN THE ATC COMMUNITY AND THE PLT COMMUNITY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT IT WAS THE FIRST TIME HE HAD SEEN AN RNAV ARR. WHEN THE CAPT STARTED DOWN AT LUXOR; HE ADVISED THE CTR THEY WERE LEAVING 12000 FT. HE SAID THE CTR INSTRUCTED THEM TO IMMEDIATELY CLB BACK TO 12000 FT. HE SAID THE TONE OF THE CTLR INDICATED THEY WERE NOT HAPPY WHEN THEY LEFT 12000 FT. THE RPTR SAID HE HAS SINCE REVIEWED ARR PROCS AND CLRNCS AND HAS A BETTER UNDERSTANDING OF WHAT TO EXPECT.