Notice that their examples are in response to some kind of objection or negative statement. This follows the pattern of using auxilliary verbs in answering objections. Here are some examples.
Bruce, you can �t be 57 years old. You look much younger!
Oh, but I can be 57, and I am.
You won �t pass the exam.
Yes, I will pass it.
He doesn �t want to learn English.
You �re wrong. He does want to learn English very much!
In each of these examples, the auxilliary verb is stressed in saying the sentence.
I hope we have helped you,
Bruce