And given the fact that people have the legal authority to euthanize their pets on demand, it seems pretty silly that we are spending millions of dollars in taxpayer money ensuring they don't get fed snake oil.
Disagree. Public money is subject to the public process, which includes debating about and voting on how resources are spent. Don't shout down people who are participating in the process.
Wouldn’t we learn a lot more about the effects of these experimental treatments if we allowed the willing to take them voluntarily?
In the case of life-extension treatments, no - we would learn much less if we allowed people to give them to their pets voluntarily.
The problem is that what you're testing for is very difficult to measure, so you need a well-run study like the one that Loyal has done here. If people just give them to their dogs without the structure of a trial, you'll never get the kind of data you need to conclusive prove life-extending effects, and because companies can now sell without such a trial, you eliminate the incentives to invest the time, money and effort into that kind of a trial.