They put them in. In the instructions they are left out, usually outlined with dotted lines though. Maybe kits that come to the USA have them but others dont??
I managed to get some decals from a coworker who builds also, so no need to send yours. Thanks anyway though. As far as why some kits dont have them, I have been told that it is because many countries in the world view the Swastika as a symbol of hate and refuse to allow it, legally, on their soil. I have a Hasegawa kit of Gabreskis P47 and the kill marks are produce in two versions. One has the Swastika covered by a white dot on every flag. The other is the normal kill marks.
aeromaster sells them by the sheet through squadron in various styles also the sheet # is 48-545. it took me about 6 months to get mine so order now if you need them for future projects (they sold out the day i ordered mine!) as for the legal issues i know that german law forbids the display in any way shape or form, years ago i built a me262 and i displayed it at the local hobby shop downtown and the owner had to put a little paper “hat” on the tail fin i apologized for my ignorance and he still gave me a discount at his shop anyway (he said i doubled his floor traffic by having finished kits in the window!) hope this helps! dave