30 Games is a bit tight, the set of SNES roms I have is 2GB and that includes the same games many times for different countries and even some beta roms
They could have easily put many more games in this.
The SOC solution they are likely using isn't an exact duplicate of NES hardware. They have likely had to do extensive testing and probably game specific tweaks to ensure accuracy. This is likely the reason that Castlevania 1/2 are there, but the notoriously tricky to get working perfectly on anything but original hardware Castlevania 3 is absent.
Also these games are being licensed from the original publishers. Konami/Capcom/Square-Enix will all be getting paid. It's not limited because of the filesize.
Plenty of games can't even really be resold these days, a lot the classic licensed Disney titles from this era would be tricky to sell these days without getting Disney on board, same goes for all the other licensed titles. As far as I can tell, all the included titles here are entirely owned by their respective publishers.
This selection of titles was almost certainly reached because they've done all the paperwork for releasing them on the Wii U eShop, so it's not much more work to do the same for this release.
Edit: I really need to reiterate that if you look at this and think there is something wrong with it that makes it unappealing, you are not the market for it, and there is a similarly priced or cheaper alternative product on the market that is likely far more in line with whatever you're after. Raspberry Pi + Emulator, Android Box + Emulator, a variety of Famiclones with cart slots, handheld Famiclones. Hell you can even get TVs with Android built in these days that would run NES games fine.
It's a novelty product for the Christmas market. It is likely perfectly adequate for that, but don't expect it to be anything more.