If a single company is doing it, that's one thing. They may be doing it so they always carry a surplus and are never out themselves. Now, if a couple companies (or more) all agreed to do something like this ~ they'd be walking a very fine line. Antitrust laws would then come in to play.
IMO ~ nothing fishy is going on.
Lets break down the numbers.
1,000,000 rounds per day from a single .22lr manufacturer
= 2,000 (500rd bricks)
Currently (from latest data) - Walmart U.S. has 4,049 stores. This doesn't include the multitude of gun shops, online firearm ammo vendors, dicks sporting goods, bass pro, LE agencies, etc... etc....
If just taking in to account WALMART, in rough estimates, each Walmart from 1 manufacturer would get 1 brick, every 2 days (roughly 3.5 bricks per week). That's not much. With Walmart's (3) total box purchase ~ a single person at each Walmart can wipe the shelves clean each week. Granted ~ we just took the 1 million number from 1 single .22lr manufacturer, but you get the gist of it.
Hopefully that helps break it down for you a bit more :)