Why isn't Selvala, Explorer Returned's ability restricted from being used as a mana ability?
The rules about ability speeds were standardized with the 6th edition. No more mention of speed was necessary on the cards after that. Some older cards were rewritten, some weren't. The 10/4/2004 ruling makes it clear that this wasn't an oversight (as the ability, despite being a mana ability, changes the board's state, while Selvala's doesn't - I'm guessing).