I'm not familiar with construction in particular, but generally speaking, employers are required to pay employees for all time they are at their place of employment, unless their employment agreement specifically states otherwise. It's not the employee's fault the work isn't ready for him to do, and he showed up.
Seems only fair, regardless of the legality of it. Imagine if it were an office with a time clock. You punch in, go to your office, and your boss says, "I don't really have anything for you to do at the moment, but I might later." Wouldn't you expect to be paid from the time you clocked in, not the time they finally hand you something to do?
Now if they're a contractor, not an employee, the reverse is true -- you're only obligated to pay them for actual work performed, unless it states otherwise in their contract. The burden is on them to have their contract indicate that they'll be paid based on arrival time at the job site, not the time work commences, if the delays are no fault of their own.