PS4 doesn't have BC with PS3 and yet my purchases of Sound Shapes and Flower carry right over.
PSP games play on Vita, PS3 games are cross buy with PSP and Vita and PS4, and yet these consoles do not have BC with one another.
This is an artificial anticonsumer boundary that Nintendo has created. A PS3 game doesn't work natively on PS4, it has to be ported, and yet Sony generally does not make you buy the port a second time. You just get it, because even if the codebase is different the consumer views it as the same product.
In the case of the VC it is literally the same product. If you buy two 3DSes you can't have your legally purchased games on both of them, and there is no BC issue since those are also literally the same product.
it is straight anticonsumer. XBOX LIVE, Steam, PSN, and iTunes figured all this out at some point in the last 6-12 years. A song you bought on iTunes in 2001 on an iPod can be on your Touch, iPad, iPhone, and iMac today for $0 after your initial purchase. If I bought Super Mario Bros in 2006 on Wii I have to buy it again in 2010 on 3DS and again on Wii U in 2012.