Regarding accepting a gift from someone who is not your friend on Steam...

Yeah, when you accept a steam gift you're automatically added to each other's friend lists

Yes, that is normal. As soon as you accept the gift you are friends.

Yes, when you accept a gift, you accept an invisible friend request. You can delete that friend later.

Yep, I bought a game one time and that happened yes.

From personal experience, it is pretty normal. I guess Steam assumed you wouldn't send gifts to people who aren't your friend lol.

It's not a bug, it's a feature. If you feel lonely, you can bribe people with gifts to become your friends. As wise people in movies say, everyone has a price :)

I'm pretty sure it says right on the page somewhere that if you accept the gift the steam user will automatically be added to your friends list. I have noticed it a couple times.