To suggest that there is "no obligation" for an employee on a zero hour contract to accept work is a bit naive.
I'm not suggesting it, I'm saying the government says just that:

Notice the 'individual is not obliged to accept any work offered' bit. What is it about that, that you can't understand?
Yeh, the employer may not offer any further work, but the employee is not under any obligation. If a contract of employment would differ significantly from this, then it wouldn't be a zero hour contract as it's understood in it's general term.
It would be something different.
Comprende?