According to what I've read, the main reason it wasn't locked is that it didn't even occur to Atari that outside developers would be able to make 2600 games. Af

According to what I've read, the main reason it wasn't locked is that it didn't even occur to Atari that outside developers would be able to make 2600 games. After Custer's Revenge (and, more consequentially, after failing to sue Activision out of existence), they locked down the 7800 pretty hard: much like 6th-generation consoles, it required games to be signed, though presumably with a weaker algorithm.