Geek! And I say that in the most respectful, Star-Trekky sort of way.
First rule of acquisition, "Once you get their money, you never give it back".
Any retailer of course free to do anything they want within the limits of legality. I have to question a retailer who throws away one of the few things remaining compared to online though. This website and others like it are case studies in word of mouth advertising. That "hobbyist" may not buy him, but they will tell some friends, and they will go online and talk about their experience. It is hard to beat first hand praise.
I must say, though, I have my doubt about a hobbyist spending "3-4" hours reviewing anything in a store. If they don’t have experience with high end audio, then this would be rather boring for them, and if they do personally, then tend to know the value of time and money and most won’t waste someone else’s.
If this is truly a concern, a more reasonable policy would be too set a demo time limit, and it could even be short, i.e. 20 minutes. This is fairly justifiable as tube-life is not infinite and replacement cost is substantial for amps such as this. If you want to listen past 20 minutes, then you need to contribute to the re-tubing fund. Any serious audiophile will understand these costs and why there would be a limitation. Margin on used gear is also lower so more justification on a time limit.
Progressive businesses thrive, regressive businesses decline and fail. In my early career I worked for two companis that tried to extract every last bit of margin out of the customer, while I watched our competitors eat our lunch over time by reducing their margin, but selling more and more ultimately making more profit and also reaching an economy of scale we couldn't compete with.
Point of clarification, you want to be compensated.
While I would accept that COVID times are different, certainly in normal times, your staff, the equipment you have on hand, etc. are sunk costs. You are going to incur them whether you have a demonstration or not.
It is also a $17,000 amplifier, It is not a $60,000 amplifier.