That may be different cultures but I find it incredibly annoying when the store clerks greet you before you've even shown an intent to talk to them, gives me the feeling of being watched all the time and expected to buy something when I'm just browsing.
I used to feel that way, and still do depending on how the interaction goes, but for the most part as a potential customer I like to feel acknowledged. I don't want or expect someone to drop what they're doing and come shake my hand and say "hey how are you today? What can I help you with" because that comes off as sorta pushy, like they're expecting me to buy something; it puts pressure on the customer. All I ask for is a simple smile and nod and who knows maybe even "Hi".
When I walk into a store and don't get greeted, and have to seek out help (which I don't mind doing) and get an employee reaction as if i'm bothering him, why would I bother coming back or giving them my business? In my experience, when you walk into a store (yes even a gamestop) and get a simple greeting, when the time comes to ask for assistance or just check out, the customer/employee interaction just comes out smoother. You don't feel like you disturbed the employee by asking for help, and the employee treats the client a little bit friendlier.
Again, I've worked retail/customer service so I've had "training" here, but more importantly just the experience from being on both sides, I know what its like. Then again, it all varies from person to person anyway so this whole post was a waste of time

Oh, and Morari brings up another important point.