You are not forced to place an infantry in the neutral - but if Germany wants to build 9 infantry, for example, one of them needs to be placed in one of their minor neutrals as Germany's max builds without their allies is only 8 infantry.
Only one infantry unit may be placed in each minor neutral per turn. So, if the Germans build three infantry one turn, they could place one in Hungary, one in Romania and one in Germany - but no more than one in each of their minor neutrals per turn. (Alternatively, they could place all three in Germany and none in Romania and Hungary). All the minor Allies do for you is increase your maximum infantry builds by one - as long as the additional infantry build is placed in the minor.
Is that clear?
I'll make the rules regarding minor infantry builds more clear as well - thanks for pointing out these confusing spots.