i am having some garden decking fitted - but think its wrong ?

I have some guys fitting some decking at the moment but dont think that it is being fitted correctly. I have a sloped garden and they have raised the decking one end and are securing to the joists to posts with screws. the other end they have not dug out the soil but have rested the joists on the soil, is this a problem? is it sufficient to secure the joists with screws rather than bolts?