Companies allowing current employees to exercise options 1+ year before a liquidity event would prevent the tax issue. For a company like Pinterest that is virtually guaranteed to have a good liquidity event, the risk of losing your principal is minimal and you are likely being paid enough in salary to afford the early exercise strike price.
If the exercise event and the liquidity event are not in the same year though, you might not have enough cash to pay the taxes. And if the liquidity event is postponed forever (e.g., market crashes like it did in 2001 and 2008), you're stuck with a big tax bill and no way to pay for it.
