ActionMailer instance method being used like class method

Here is a brief snippet from the guide on ActionMailer

    class UserMailer < ActionMailer::Base
  default :from => "notifications@example.com"

  def welcome_email(user)
    @user = user
    @url  = "http://example.com/login"
    mail(:to => user.email,
         :subject => "Welcome to My Awesome Site")
  end

end

And in the controller

class UsersController < ApplicationController
  # POST /users
  # POST /users.xml
  def create
    @user = User.new(params[:user])

    respond_to do |format|
      if @user.save
        # Tell the UserMailer to send a welcome Email after save
        UserMailer.welcome_email(@user).deliver

        format.html { redirect_to(@user, :notice => 'User was successfully created.') }
        format.xml  { render :xml => @user, :status => :created, :location => @user }
      else
        format.html { render :action => "new" }
        format.xml  { render :xml => @user.errors, :status => :unprocessable_entity }
      end
    end
  end
end

So why is Rails trying to confuse rubyist with instance methods as class methods? I assume they've overridden missing method, but it just serves to confuse! Or am I missing something here?

ie why not define welcome_email as def self.welcome_email(user)