that's how 99% of belly heat racks are, the only belly heat racks that aren't that way are ARS and Freedom Breeder racks and those cost thousands of dollars.
As long as you check the flexwatt even once per year, there is no problem with having the tubs directly on the flexwatt, in fact it is necessary for them to be touching otherwise you will have to run the flexwatt far too hot to maintain the proper temperatures.
in the case of ARS and Freedom breeder racks the flexwatt is in a metal casing and the tub sits on that piece of metal. but using the metal has it's own issues (suck as risk of electrocution)