Every EULA is different, and it might be different in other country, but in Canada, EULA are NOT enforcable. Ill give two exemples:
Boxed product. If you buy any kind of electronic product in a store, lets say, gameX from Microgame, bought at a local SellersY, the contract would be between you and SellersY. The EULA you have to accept from Microgame to install the game would have no legal validity, as you already exchanged money to get the game. SellersY, not acting as a Microgame agent, is the one having a contract with you.(In Canada, ANY money exchange is a contract, even geting a haircut) If Microgame has any dispute with this process, they will have to take legal action against SellersY.
Regarding an electronic purchase, the EULA can be legal but only if those two conditions are met:
1-The EULA is available BEFORE purchase, ans must be signed(and we mean SIGNED) A *I agree* button is NOT an electronic signature under the law unless...
2-You can keep an unalterable copy with your signature on it for your own archives.(A *X* can work as a signature, as long as YOU put it there)
These two clauses render most EULAs invalid, as you can easily buy games without any kind of EULA(they usually show up when you install the game, NOT on purchase). Also, any EULA that get updated(that actually change the EULA documents on your computer) is not legal binding as you bought a product under the OLD EULA. You could accept the new EULA, but even if you refuse, they cannot prevent you from using the product, as BOTH parties must accept the new contract for it to takes effect.
Regarding the U.S.A, some states(I tried to find a list, but I could not find one) do have laws stating that EULA CAN be enforced.
Finally, regarding Steam account being made for advertisement, I think it is a great idea. I much prefer having directed ads linked to my interest than mass general ads regarding product I dont care. (Tough, since I am working in the advertisement business, I might be biased
)