I wonder if the GTS copy glitch has remained constant up into X and Y. If that's the case, then it would explain as to why these are found on the GTS now.
The idea is that, when you want to get a copy of you Pokémon, you put it up on the GTS, then when it is halfway through saving and stuff, you turn the game off. This will result in the Pokémon residing in the space it was in your team/PC, whereas there's a duplicate of it on the GTS in your offer slot. But because it is up for offer, anyone can get it, should they have the right Pokémon for trade. This is why they ask for the impossible; any legendary at Lv. 9 and under. This will promise them that the Pokémon that they copied will still be there when they go to get it.
Again, this is only if the glitch still exists in these new games. If not, then yeah, they're just being trolls.