To avoid too many suckpuppets and bring more serious users. The giveaway is very large amount and you don't want one person sign up for too many shares. 0.0001 and even 0.001 are almost free and are not big deal. Because it is a hassle to do so, it can prevent many suckpuppets.

I guess that because NFD wants to give each user a decent and large amount of coins for free or almost free, NFD wants more serious users to participate.