Because the cookies are a rip-off, yet the consumer is just supposed to roll over and take it. They are cute little scouts, after all. Now, if the GS were sent out to solicit donations, and in return they gave away the cookies as a "thank you for your support", then it wouldn't be deceptive. The scouts would not have any illusion that they were selling a product. Yet, they are dispatched ostensibly to "sell" cookies, as if normal supply and demand would support those kind of prices.
Many of these kids simply have no concept of real-life values of products or services, but they (some, not all) do learn to cop an attitude when they hear the "no" word. Or, they learn (from the attitudes of adults around them) that the people who won't buy the cookies are just stingy scrooges.
This is a general gripe I have about fundraisers that involve kids. The people in charge of these schemes are happy to use kids, because consumer-victims are generally unwilling to turn down children. I just don't like it. It is smarmy and manipulative. To each his own.