Are M3/R4 flash cards for the DS illegal?

Question

I mean the actual hardware, not using them for ROM emulation etc just the cards alone and by themselves.

Answer 1

The cards themselves are totally legal, because they can be used for legal homebrew purposes as well as backup purposes.

Answer 2

as long as you are using it for homebrew purposes only, you are okay. Homebrewing is actually a great way to better understand a games system, but that is off topic. so, no, as long as you are  not using it for ROM software, you are ok in that section.  

Answer 3

I believe home brew is illegal because developers have to pay a fee to Nintendo for releasing their programs and making money out of them.