1) As said, it's not just anyone. All thouse who have become wights have been killed in White Walker situations. All the slain around Crastor's Keep after that little dust up didn't reanimate, or at least not in a hurry, having been killed by mortal men. Now, had a walker shown up, perhaps it could have made use of thouse corpses.
2) Of course, the reason they're doing that is because it's super cool, naturally. They've explicitly said it won't take place in the books, which is natural since the books benifit less from that sort of cinimatic drama. But, if you want the Magnificant Seven with swards and zombies, then this is what you get. It's a bit less Game of Thones-y and more TV-y, so there's no small part of me that doesn't really like it, but at the same time there should be a lot of awesome fighting and so on, so there's a medium sized part of me that's relitively excited.
Now, I just hope they're well prepaired. There's really no point in assembling a small, elite strike force and not prepairing them for the job correctly. So if they don't each have a belt full of obsidian I'll be a little put out. They should also have pretty excelent intel, since Bran sent a fleet of spy drones in to do airial recon, so if (when?) they blunder into the enemy unexpectedly I'll again be a bit shocked.
