Some people do want the combo, but they just say the sandwich or the entree, whatever. I can't tell you the number of times I've had someone say they wanted a sandwich (mentioning nothing about the combo they just say the sandwich) and then get pissed off at me because they wanted the combo. Apparently we are supposed to be mind readers. Sorry to say, a mind reading capability is not part of the orientation package of any fast food joint.
You may mean that you just want the sandwich or the entree, but we have to ask just in case. Besides the fact that the company wants more money so wants us to suggestive sell.
Now, at Taco Bell, if you say to me that you want the Nacho Bellgrande, that's what I'm going to ring up. I won't ask you if you want the combo because you just said the Nacho Bellgrande. I will, however, at the end of every order if you haven't already ordered one ask you if you want to try a fruitista freeze. It's annoying as fuck for me to say, but I have to say it. There are so many areas where I would lose points or get in trouble if I don't suggestive sell it.