According to the e-Reader cards for Mario Bros. and Ice Climber, Tom Nook gave these away for free as a "loyalty gift" for customers of his various stores.
#N01 Ice Climber
Some people will do anything to get you into their tents, or stores. Well I can tell you I will not stoop so low. I did however find this classic NES game on a storeroom shelf. Would you like it? Let's just call it a "loyalty gift" shall we?
The original Japanese release of Animal Forest+ for GameCube lacks Soccer and Excitebike, and instead features the early Famicom versions of Gomoku Narabe and Mahjong.
In addition, the various Famicom games (Save for the Famicom Disk System) are given a color code as well:
Orange: Clu-Clu Land
Grey: Balloon Fight
Red: Donkey Kong
Blue: Donkey Kong Jr. Math
Light Blue: Golf
Depending on the version of the game, some NES games are obtained through different means.
Clu Clu Land D, Soccer, Donkey Kong 3, Donkey Kong Jr. and Punch-Out!! were initially unavailable in-game, however Nintendo of America and Nintendo of Europe later released codes that could be used in the game to unlock them, though Punch-Out!! was deciphered by fans.
Super Mario Bros. and The Legend of Zelda cannot be played through normal means, although Super Mario Bros. was given away between 2001 and 2002 by Famitsu for Animal Forest+.
Some NES games were re-released in Classic NES Series for GBA.