TMs: Basically CDs that contain information for moves for compatible Pokemon.
What happens to the old ones? Ones that were used years before?
Say the Gen 2 TM for Zap Cannon was never used... what happened to it? Surely the owner wouldn't just throw it away. Maybe he decided to store it, and maybe, a long ways later, he caught a Pokemon. A Raichu, for example, which was compatable with said TM. Say he decided to teach it to the Raichu... would this be allowed? Because even two years ago, there was the TM for Rock Polish and Dark Pulse... now... they're gone.
They were probably in high demand and probably many are still in circulation.
So why not allow older TMs (as long as reason is given) to be taught to compatible pokemon? Given of course that they fit the Generation of the move. E.G.: Zap Cannon couldn't be taught to Manectric, as it wasn't part of the Gen to be compatible with the TM.