UNAUTH RWY ENTRY BY ACR.

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TWR TOLD US TO EXIT RWY 18R ASAP SO HE COULD LAUNCH ONE PRIOR TO THE NEXT ARR. THE FIRST AVAILABLE TURN OFF ONCE WE REACHED TAXI SPD WAS RWY 27/9. WE TURNED THERE AND TWR DID NOT APPRECIATE IT. WE WERE COMMITTED HOWEVER; ONCE WE STARTED THE TURN AS TWR CLRED THE TKOF POS ACFT TO GO AND HAD WE TURNED BACK ON THE RWY 18R TO EXIT AT A TAXIWAY; A REAL CONFLICT WOULD HAVE OCCURRED. RWY 27/9 WAS CLR OF ACFT. HOWEVER WE DID NOT HAVE SPECIFIC TWR PERMISSION TO USE THE 27/9 RWY AND THAT MADE THEM MAD. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. PIC DID CALL THE TWR SUPVR BY PHONE TO REVIEW INCIDENT. THERE HAS BEEN NO FAA FOLLOW UP. AFTER TOUCHDOWN THE TWR LCL CTLR SAID 'ACR X TURN OFF RWY AS SOON AS POSSIBLE.' RPTR AND PIC COULD HEAR TWR CTLR PUT ACFT IN TKOF POS BEHIND THEM ON 18R AND ALSO KNEW THAT THERE WAS ANOTHER ACFT ON FINAL SOMEWHERE. THERE WERE NO ACFT USING RWY 9/27 AT THAT TIME. WHEN ACR X MADE L TURN ONTO RWY 9/27 THE TWR CLRED ACFT Y FOR TKOF ON 18R IMMEDIATELY. PIC OF ACR X WAS LOW TIME IN TYPE AND FIRST TRIP AS CAPT. THEY WERE TOO FAST FOR TAXIWAY D4 EXIT SO PROBABLY SHOULD HAVE CONTINUED TO TAXIWAY 'M'.