Even without a moral obligation, there's a pragmatic reason to spend money wisely: if you blow the money on frivolous creature comforts, you end up in the same state you were in before the startup (which usually means poor), except that nobody will invest in you again. This seems like an objectively worse position to be in.
The real winners when startups blow all their runway on fancy perks are purveyors of luxury goods. That's where all the money goes, after all.
I've seen start-ups that blew all their money in ridiculous ways - to me - without ever crossing that line. You can burn a lot of $ once you start shopping in SF for hip office space, create an irresponsible marketing budget and hire some expensive VP's of something or other. And VCs will applaud you all - or at least most - of the way when you do that.
I see venture capital in the United States exactly the way I see record companies: for the most part they try to get the noob's signature on the dotted line on a contract they will wish they never signed a few years later on the off chance that they too will end up like a few of the mega stars did.
The real winners when startups blow all their runway on fancy perks are purveyors of luxury goods. That's where all the money goes, after all.