See if you can follow the logic for a moment. If I can click a button and get the same service for $25, is it reasonable to assume Verizon can provide the service for that amount? Then your argument holds no water.
Yes, it is reasonable to assume that Verizon can provide the service for that amount. It is ALSO reasonable to assume that Verizon MAY have a similar agreement with international providers where they are charged a much lower rate IF they contract for the service BEFORE being used, just as Verizon requires you to do so.
See if you can follow the logic for a moment. YOU subscribe to the service, Verizon lets the international provider know about it(i.e. Verizon contracts for your line to utilize service with the international provider at a lower rate), Verizon gets it at the lower rate and passes on the savings to you. YOU don't subscribe to the service and Verizon isn't able to notify the international provider(i.e. Verizon does not contract for your line with the international provider at a lower rate), Verizon does not get the lower rate in order to pass the savings on to you.
Of course, I have no way of knowing if this IS how the agreement between Verizon and international providers work. I ALSO have no way of knowing this ISN'T how the agreement between Verizon and international providers work.
Regardless, when travelling internationally, you will always get lower priced service if you simply remove your Verizon SIM card from your phone and replace it with a locally purchased SIM card from a provider at your destination of choice.