Use of Will/Shall

[Winwin2011]

The followings are quoted from a Grammar textbood.

We do not use "will" for things we have arranged or decided to do.

1. We're going to the cinema on Saturday. (not 'We will go')
2. I'm not working tomorrow. (not 'I won't work')
3. Are you going to do the exam? (not 'Will you do')

Do not use I'll......for something you decided before

1. I'm not working tomorrow (not 'I'll work')
2. What are you doing at the weekend? (not 'What will you do')

For 2. What are you doing at the weekend? If I ask John "What are you doing at the weekend? I do not know whether John have arranged or decided to do anything at the weekend. or even John have not arranged or decided to do anything at the weekend.
So, why can't we use "What will you do at the weekend?

Thanks for your help!

[5jj]

1. I'm not working tomorrow (not 'I'll work')
2. What are you doing at the weekend? (not 'What will you do')

For 2. What are you doing at the weekend? If I ask John "What are you doing at the weekend? I do not know whether John havehas arranged or decided to do anything at the weekend. or even John have has not arranged or decided to do anything at the weekend.

If you are asking the question you are implicitly asking whether John has arranged anything.