There were never official NES/SNES or GBA compilations for Dreamcast. Some homebrew programmers programmed emulators for various systems in order to play NES or other ROMS on the Dreamcast. ROMS are illegal if you don't own the original game. It's like renting a PS2 game and make an ISO. I have ROMS of all my old (pre-PS1) games so that I can play them on my laptop or take them with me to a friend, but I don't have any ROMS that I don't own the original game to. To me it just feels wrong, like stealing. And it takes away the fun (and pride) of building your private game collection because downloading the ROMS is much easier and less expensive than searching eBay for games you'd like to play.
So, if people see these covers for emulated systems, they might feel inclined to download ROMS. Some of them might even hear about the possibility of emulating other consoles on DC here first. So I'm with SMMM on this one. Maybe not banning the covers but clearly dissociate from (illegitimate) emulating.
The Half-Life for DC is another thing. The game was finished, ready to be shipped and just a few weeks from release they decided not to publish it. Somehow a (press?)-version was leaked on the net, fully playable on the DC.
I know this is a double standard but I wouldn't consider owning that game stealing as it was never released in the first place. There was no limited release, no demo, no nothing, there was NO way of getting that game other than downloading it even if you WANTED to buy it. Basically, it could be considered trash, the company wouldn't release it and threw it away. Yes, this is legal limbo but I definitely prefer (and understand) the people who download Half-Life for Dreamcast to the criminals who download PS2 or Xbox360 ISOS.