The Premier Ball commemorates the fact that you bought 10 Pokeballs, helping the Pokemart in their business. As said in your quotations:
"A rare ball made in commemoration of some event."
"A rare Ball that has been specially made to commemorate an event of some sort. "
"A somewhat rare Poké Ball that has been specially made to commemorate an event of some sort. "
"A somewhat rare Poké Ball that has been specially made to commemorate an event of some sort. "
The Event is the buying of the Pokeballs.