Are you kidding me?
No, they can decide if YOUR files in the folder (that it's signed) are bootable by their system or not.
But they can't check the entire sd looking for suspicious files... And they can't ban your for that reason.
It's quite incredible to me how you can keep defending your stupid opinion without even considering whether your belief is true or not.
The law you are talking about does not exist. There is no legal concept of "your" files on an sd card. You are willingly inserting the card into a Nintendo device. That device can read your memory card as much as it wants.
Link to a law number or law text that supports your opinion or shut up.