Earlier quoted context omitted.
It's not that it's legally permissible, it's that it's within bounds of what all parties agreed to. If I agree to sell you my car for $5,000 then you show up with the money and give it to me and take the car, I don't get to change my mind later and get mad that you didn't give me more and call you immoral.
I don't get to change my mind later and get mad that you didn't give me more and call you immoral. But on the other hand, if you’d agreed to sell me your car at a price under what you might be able to get elsewhere—maybe because I was financially struggling and you were doing me a friendly favour—you’d probably be pretty miffed if I won the lottery and still demanded you stick to your offer…
They chose not to, which would seem to indicate they weren't miffed. Nobody forced them to do anything. It's not immoral to ask someone for an accommodation and accept it.
The investors didn't give him the money as a friendly favor to help him it out. It was an investment. Presumably they make a lot of them and understand the risks involved in doing so. If you're going to get hurt feelings whenever one of your investments doesn't pay off, you're not cut out to be an investor.