I kinda agree with u there.
That yes it is an obvious extra way for them to make cash.
But it is optional meaning that if you don't use FS rounds or simply prefer been able to carry 2 types of ammo in 1 mag..you still can, but without paying extra for mag internal parts that your not going to use or need.
So I think these sort of extra/optional d-mag configurations were inevitably going to be released.
But I also think these optional d-may components might be welcome in the massive variety of playing styles out there.