Blame it on Apple, they mandate using this button. I understand why they don't want to use prices, because those are localized, but agree that it should differentiate free vs. paid apps.
Odd, I could have sworn they provided a "Buy on the App Store" or similar but I just read through their marketing guidelines and you are right. Well then part of my argument doesn't hold up but still I feel like developers are doing this on purpose (obstructing the price and/or the fact it is paid not free) to hide that from you until after you get excited about the product/app.