I think the reason for this is because then you aren't paying the extra $30 for original PA. Basically steam making sure no one tries to use a loophole
Not with a upgrade price, since you can't purchase the upgrade as a gift. Since it's a upgrade it makes sense it can't be gifted.
Titans on sale https://www.humblebundle.com/store/planetary-annihilation-titans?hmb_source=search_bar