I think you all mistake my words here. I'm not even suggesting they're illegal, far from it. Bootleg might have negative connotations, but in electronics there's nothing illegal about them. Creating, selling, buying, or using bootlegs is perfectly fine and legal. Say you have a game app on your phone, but not one from the Play Store. Then said app is a bootleg as Google didn't approve or support it, doesn't make it illegal despite what Google might think, its just one outside their catalog.
Same for 2600, the games are bootlegs if Atari, Sears, or Coleco didn't release them, but that doesn't make them illegal in any way. ...thought I must say I think Fire Fly is pretty criminally bad. Don't read 'bootleg' as 'counterfeit.' Though some companies use them interchangeably, they're very different.
By your definition(and no one said "illegal") Activision games are "bootlegs".
The point I was trying to make is that First Star Software endorsed Boulderdash for the 2600, thus making it an "official" release even though no one on their staff coded it. Halo2600 was coded with, I'm guessing, a "gentleman's agreement", as I never heard of there being an actual contract signed(correct me on that if I'm wrong). Of course, it's not like a game released for the 2600 is going to cut into sales of modern games.
Homebrews, in my mind, are games created by independent coders, who don't do that sort of thing for a living. Yes, Boulderdash was a labor of love, just like other homebrews. The difference was the contract between the coders and First Star Software(the copyright holders).
Except for Nintendo, I haven't heard of any C&Ds being issued over Atari console homebrews. So, we can reason that other copyright holders don't feel it's worth the time or money to take legal actions for games on this level.