There are a number of shipping options that dont require entering address info on the cart screen but if you are using the real time shipping calculator it requires address info on the cart screen. I do see what your saying if you use the shipping calculator that the sign in page should come up when you click the add to cart button. But looking at the issue from a different angle directing the user to the sign in page when they click the add to cart button could be kind of annoying for the user as sometimes customers like to jump to the cart screen quickly to see the cost of the items in the cart and the subtotals for shipping, tax, etc.
-Scott