Hello Lane, welcome to the gaming industry!  I think your questions regarding the practice of companies selling new product to the mass market early and cheaper are good ones.  To me, the answer is a short and depressing one.  In short, it's because they can, it benefits them, and they don't care about you.

Here's why.  By selling to the mass market a company gets a huge sale and nationwide exposure for their product.  If the product does well and there is a demand, they know that game stores will beat the manufacturer's doors down to be able to carry the product.  In the end it doesn't matter how much a store owner stomps and complains about this policy, if a store passes up too many hit games they will be out of business.

Mass market stores operate on an extremely thin margin, maybe 5% to 10%.  In addition they do their own distribution, so that cuts their cost.  By the time all is said and done their selling price is equal to your buying price.

In addition, mass market nation-wide stores know they are an extremely attractive client to a game manufacturer.  They use this as leverage to gain perks that game stores can never have, such as early release dates, and guaranteed buy-back on unsold product.

Sadly, I think this is the case of game stores building up an industry over the past 10 years, then the mass market stores stepping in to claim the market game stores worked so hard to build.