EM compatibility of single board computers and laptops
It's all about the level of (well the quasi peak level of) radiated emissions.   If you can design a card that doesn't exceed them without a shield then you don't need one.  In high volume consumer electronic design your mission is pretty much never to use a shield (well maybe for an rf section).

Yes that means doing your design carefully and considering the impact of your decisions on your EMI.  It also often means making use of a spread spectrum clock to take advantage of the way emissions are measured.

I would wager that some sbc boards you can buy have done the work to ensure they are compliant, and other cheaper knockoffs may just put the logo on and believe they'll never get caught...