Well, it might be strange to you to pay half upfront, but what about the contractor's point of view?
HE's working for folks here in the DR too, and once he does the job, how do you think HE can get paid if the customer decides to burn HIM? Can he get back in the property , and repossess the work and materials he just got done installing??? (that's tougher than tracking someone down to come and finish the job, isn't it?)
I have no doubt that the contractors here do this for a reason, and not just because they need materials or live on a tight budget.