That's because they are an advance payment for services, thus are correctly held on financial statements as prepayments.
Security deposits at Apartments are not pre-payments.
And for purchases/sales, profit/loss on trades, etc.
If you are going to commit to selling 1 BTC for $600, they need to know that you have the 1 BTC, and then to withdraw the BTC when the trade is executed. Like it or not, they have to have control of your BTC if you want to transact business. The question is whether they control the BTC on their books (and thus shows up on their balance sheet), or whether a 3rd party holds them and does the transactions at their direction.