I think they would be foolish to Sue Walmart. For one, regardless of whether or not they jump the gun, Walmart is NOT the one who puts the product out. Walmart only provides space for the Yugioh TCG Products, and depending on what store you go to, there may not even be that much of a selection for the simple fact that most Walmarts would rather make room for product that THEY control.
Yugioh is not stocked by Walmart. If you ask employees when the new set is coming out, they will normally give you a blank stare since they have no product control over it. Again, they only provide space for it to be sold in their Stores. Its like going to a Swap Meet. The actual Store or Lot that sponsors it, is not the one who controls the sellers or their products. They just give them a spot to sell their goods from.
So, UDE can sue Walmart all they want. The only thing that will happen is that they will just stop selling it, the small towns that Walmart is the biggest store there, Yugioh players will be unable to have access to new cards, and will be unable to go other areas to buy them and that will cause people to leave the game.
Even losing "one" player for something as ridiculous as Walmart beat everyone else to the punch is stupid. What usually happens when everyone here's the latest set is out? You dont see them saying,
"Hey, Im not buying it cause it said it won't be out for another month!!!"
"I want to be fair to my buddies and wait to get it with everyone else!"
If you are one of those people who would do that, I commend and salute you! For the most part, I know 90% of you will try to get to where ever it is that is selling it so that you can get your hands on them before they sellout. I told everyone that the Warrior Structure Deck was going to be released in mid-October, but everyone swore it wasnt going to be until November. I had first hand info from the person who Stocks Walmart with Yugioh. I used to be there "Johnny-on-the-spot" the day they would tell me to be there.
UDE isnt going to cut off their nose to spite their face on this. Its still making money regardless. We all know the better cards come from Hobby Boxes, so those people who dont have any Hobby Stores in their town, who cries for them?