I've spend some time to investigate this issue and discovered something strange.
"Process settings prevent this task.." appears when "Must open the form to complete the task" option is checked in Assign Task step.
When process is invoked for the first time and this option is unchecked the process fields appear (I checked it on Ipad and Iphone mobile client).
Once any change is done on this process, the process fields disapear from the mobile client and there is no way to approve/decline task only from the workspace.
Did anyone encountered this behaviour?