Beyonce is Beyonce and she can get away with selling something exclusively through iTunes for a week and have lots of success
. The problem she is going to face now is that there are very few places people can actually buy a physical copy of her CD. Target already announced they wouldn't carry it and Amazon joined in this weekend. Both retailers are ticked off at what she did. So, even though Beyonce had record sales this week, she knows the importance of the physical copy still which is why she popped into Wal-Mart who is selling the CD.
What about other artists though? They might think they can have Beyonce numbers and sign an exclusive deal with iTunes and then be screwed when that doesn't go well and then consumers only have Wal-Mart as their main sales option. Yes, independent record stores may also have them and I encourage you to shop at them but they generally only have one or two copies of each CD. Even though you can buy the CD on the Amazon site it is through third party retailers and Amazon will not display the CD unless you search for it by title and spell it exactly right.
In the next few weeks I think you will see Beyonce's sales numbers drop much more rapidly then they would have if she didn't do the deal with iTunes.