It's more cause and effect.
The majority of the Sonic fandom online likely isn't key to this, whereas the majority of the target audience is. The majority of money SEGA earns off Sonic comes from Nintendo Releases, the majority of their target audience (Young kids to early teens) play mostly on the kiddy/casual/whatever console. It's highly likely as we transition to the next console generation that SEGA's profits would indeed still come from Nintendo, it is pretty much a gambit that will pay off for SEGA. Meanwhile other games that SEGA are developing have a target audience that has performed well off Nintendo consoles but perhaps poorly on Nintendo consoles, as such will have development for the other consoles of the next gen.
It's marketing as daft as it sounds. People are basically complaining that SEGA's putting their stuff where they get most money, I assume these people also don't understand how businesses that work for a profit work?
Also Hogfather, I honestly really would not measure game success off our country. We're not really a good barometer for it at the moment, Europe as a whole sure, but the UK Videogame Industry is suffering a little from poor sales in general here >_>;