I agree with both of you but perhaps, they had to keep a certain amount in their hot wallets so they could process fast/priority withdrawals [not taking sides].
- Unfortunately, they paid the price for offering such withdrawals [even though I'm sure they've gained a lot from those fees] and it'd be interesting to see if they'll be offering it again.
Trust me, there is no need to keep that amount in a hot wallet, although I don’t know how much FP pays on a weekly or daily basis to its customers, but I believe it is even much less than 5 BTC in total. This is nothing more than a simple irresponsibility towards oneself and customers - and the only positive thing is the fact that the service has remained active, because in the past things like this usually meant that everyone could say goodbye to their coins.