It's a pretty sucky rule, and something id expect from gw. Won't be buying one anyway. They don't even MAKE a 6x3...
The irony is that when GW made battlefleet gothic, in the states they made a felt 8x4 star map that retailed for about ten dollars.
I know because i bought two for myself and green flock and sand flock one (on felt base) when i was running regional tournys as we needed a couple of hundred for UK regionals and the UK company didnt sell them so we ordered a load in from the US.
(like many firms GW is actually a dozen or so seperate businesses and in UK Studio we had to 'buy' stuff from manufacturing or from 'GW retail US' etc... you couldnt just go, 'please post me 200 mats'.)