The WT Relief work as I understand it is this.
A disaster strikes for which the resident has insurance, be it a hurricane, wildfire, large hailstorm, or whatever.
In such cases, local contractors are overwhelmed, so it is not unusual for contractors (JW or non-JW) to move in temporarily for the work. There is usually a demand surge on materials as well, resulting in delays. JW's take in mostly donated materials, donating their time and expertise for the rebuilding/reconstruction work. They feel like they are doing good for the brotherhood and get that happified feeling. The JW homeowner is just happy to get the work done and get back into their home. Then when the homeowner JW gets the insurance check, they turn it over to the WT.
I don't think what they are doing is illegal or fraudulent, though it seems kind of underhanded to me.