Probably the founders of AirBnB would agree that theirs is a risky business, which is one excellent reason to take some money off the table.
Think of it as hedging your bets, on the one hand you keep a large chunk of stock because the company may weather all the storms ahead successfully, on the other hand it may not. The insecurity can be translated in to a strategy where you remove some money now in case the 'worst case scenario' becomes a fact.
Nobody is forcing any of these guys to do business under these terms. If the VC doesn't want to be part of the round then they should just pass on it, it's not like there is a guy with a gun behind them making them ink the contract. If they don't like the terms, don't do the deal.
Think of it as hedging your bets, on the one hand you keep a large chunk of stock because the company may weather all the storms ahead successfully, on the other hand it may not. The insecurity can be translated in to a strategy where you remove some money now in case the 'worst case scenario' becomes a fact.
Nobody is forcing any of these guys to do business under these terms. If the VC doesn't want to be part of the round then they should just pass on it, it's not like there is a guy with a gun behind them making them ink the contract. If they don't like the terms, don't do the deal.