Um, actually, thats become how spyware companies get away with their businiess. They develop an "anti-spyward" program that installs with their other software, this way, when another program removes it after detecting their spyware they can sue the company that made the product because it is limiting competition or something along those lines.
Yahoo toolbar and Google toolbar aren't spyware, although a lot of times it seems that way, but you have to agree to install it before it will go on, so the user has a choice. So if you install Yahoo messenger, if you don't choose to not install the Yahoo toolbar then you'll get it.