Well, there's a set of reasons behind it, but it doesn't have to do with working for free. Lawyers are allowed to do that (pro bono work).

It really has to do with establishing the attorney-client relationship. Legally, it's established when the client thinks it is, regardless of what the lawyer thinks. There is a certain standard of reasonableness, but courts will often come down with decisions that appear anything but reasonable. This makes most lawyers leery, particularly since the cases they most often study are the outlandish ones, and most lawyers are not experts in legal ethics (heh, heh). If a lawyer-client relationship is established, it can be difficult for the lawyer to terminate it, even if they're not being paid or the client is difficult(it could involve an appeal to a judge if they're in a current case).

And, yes, you're right, it's probably not necessary here. But lawyers get used to using a LOT of extra words, and typing a disclaimer is a small effort. Also, a small part of it was kind of a joke, particularly the reference to privilege (clearly here it should be obvious that the communication is not private).