Eh, beats me. I think the
LocalFriendCodeSeed_B has more to do with online play and the
movable.sed (with a linked NNID and/or Nintendo Account) deals with purchasing digital goods on the eShop.
We can speculate a number of reasons why Nintendo doesn't outright ban you for using a public seed that's not your own.
Perhaps:
1. Their ID checks aren't very stringent for the 3DS. Nintendo might not have thought cross checking the *.sed and seed were necessary security measures.
- You can see they learned their lesson and wisen up for their Switch. That shit is on Fort Knox level of lock down in doing anything not explicitly allowed by Nintendo.
2. Nintendo does not care that much about people with custom firmware on their 3DS at this point in time. *
- The 3DS is close to retirement. Why put that much effort in hunting down homebrewers so long as * they don't rock the boat cheating online?
3. Even if some 3DS CFW users pirate their games, Nintendo does not want to ban you from their store if you still legitimately purchase their goods.
- Money is money. Why turn away good paying customers?
There's also the (serial number) super ban where LocalFriendCodeSeed will not save you. This type of ban is reserved for special occasions like reported stolen 3DS systems or customers who conduct business in bad faith doing a credit card charge back.
Sorry, I'm not that knowledgeable about this aspect of the 3DS, but I hope that answers some of your questions.