I don't think it's about banning words at all. It's about words making sense.
"What's cheaper? The price is." Now that just doesn't make any sense, since a price isn't cheap or expensive, it's high or low. The thing that is priced can be cheap or expensive, but that's not what's being said.
"What's faster? The speed is." Doesn't make sense either. Speed isn't fast, the speedy thing is. However, "What's faster? The acceleration is." is fine, because you can have slow or fast acceleration (I think?).
I'm an ESL speaker, so please do tell me if I'm wrong and how.