Dude, you've been cheating for a long time. 6th ed. BRB, under Disembarking pg. 79:6th ed fast vehicles could move 12 and were considered to have only moved at combat speed so they could disembark after that move and then assault.
Nothing about Cruising Speed, and if we mosey over to Fast Vehicles (6th ed. BRB, pg.83) we find that the Cruising Speed stipulation has only to do with firing weapons, not disembarking.A unit that begins its Movement Phase embarked on a vehicle may disembark either before or after the vehicle has moved so long as the vehicle has not moved more than 6".
There's tons of stuff I have had to check out as things move from edition to edition, I got back into the hobby with JUST enough time left in 5th edition to collect a Razorback-oriented BA army only to have everything I had spent my money on basically invalidated. Since then I have had the current BRB within arms reach at all times :laugh:funny that I actually started doing it a long time ago because of a local tournament where I watched it happen and the guy actually came 3rd