What is the reason behind "no cheese" on top of seafood pasta ( think pasta al tonno e alici)

I don't think its anything like chemical reaction. Its just two flavors that do not marry. With a seafood pasta, you'd want some zesty/citrus feel to it, not so much the cheesyness on top. Then again, taste differs. Some people might love their seafood pasta with cheese. But most foodies will consider it a big NO NO, just purely taste wise.

It's an Italian cultural thing. There are some exceptions but very few.

Here's a short article on the debate but I'm a believer of "if it tastes good do it" :
http://www.thekitchn.com/the-fish-and-cheese-debate-the-76178