Herein lies the problem with zero hour contracts. It can be written by the employer to seem fair, no obligation from either party. As Tish pointed out, by definition that is how they are written.
In reality they can be used by an employer: you work for me when I say or no more work for you sonny bob.
For window cleaning it is ideal. Weather issues can mean long periods of no work, cutting out the wage bill is a massive overhead gone. I have no problem with zero hour contracts, most employers are fair. Some however are not.