So generally speaking... No, they do not hold GA tickets, or any tickets, specifically to get people to buy other seats and then drop them later on.
That would be illegal.
When tickets go on sale, a certain amount of holds are put aside - band, promoter, management, arena staff, sponsors (of the tour, the promoter and the venue), fan club holds, contest holds, etc etc. There are also tickets that are bought through fraudulent means (stolen credit cards, etc) that are returned to the pool, but these are relatively small.
If any tickets are left in the pool come show time, they are given back to the box office/Ticketmaster (depending on how late they're kicked back) and released to the general public.
This is why sometimes drops are plentiful, and sometimes they're a trickle, non existent, or ABSOLUTELY last minute by stopping at the box office after the opener has already gone on and stumbling ass backwards into one.
Depending on the show, location, etc. half the seats in the arena could be spoken for before a single ticket goes on sale in the general.