They also have to visually check the condition, and if you come to the store, at least there's a chance you'll spend some money there, or trade your stuff because you're 'already there'. Whereas if they give you price over the phone, not only can you then say 'You said five bucks!" when your game looks like it's been through the shredder, but that's 'non productive' time--if I'm a customer at the counter ready to drop 35 or 50 bucks on a new game, and he's on the phone giving trade prices for someone who is not likely to even bring them in and trade them, I'm going to be pissed, and they may lose *my* sale in addition to any business you aren't bringing them.
It's a business decision, they're trying to maximize profits and focus on their actual customers. I don't have a problem with that.
Next time you go to EB, just take them in and ask what you'd get for them. They should have the display set up so you can see what they ring up as. You can always say 'no thanks'.