I think you are right. The founder thinks: "If I don't spend all their money, they will think I raised more money that I needed, which is probably true, but they don't have to know."
While the investors think: "If they don't spend all our money I will think they are lowing the chances that this investment will pay back, so they better spend it all".
One trend I have noticed a lot in Berlin is money being spend in all sorts of things that are in no way related to the product or service the startup was trying to create - and the few times I dared asked I got the same "they are investing a lot of money on us man, that party should better show it!".