According to GPL term and conditions once a software released under GPL license can be used and redistributed without restriction.
The following statement: "Our components are licensed to be used on ONE website only." does not comply with GPL term and conditions.
Are DTH products released under GPL, in which case I can buy a subscription to a product and use in on how many sites I choose to; otherwise the following statement is false: "Distributed under the GPL license"
With GPL, we can not CODE any restrictions... in the code, there is no single site limitation. Can you install the component on other sites? Yes... as has been stated in various places in this site/forum. The limit comes in that fact that we will only provide support for the one site you purchased the component for. That does not violate GPL at all... and many other developers do the same thing.