The source of the problem is liquidity, not the problem assets. But providing this liquidity costs money. And, in my book, that money should not have been spent by government. Here is a simpler example:

A person deposits $1000 at a bank at 1% interest. Bank buys 10-year treasuries yielding 3%. Rates rise, treasuries drop and the person suddenly wants his $1000 back (say, because he can get 4% at an MMF with checkwriting privileges). Bank can sell those treasuries at $950 and eat the $50 loss. Or use own money. Or get a loan.

But this is not simply "the bank just needs liquidity". It sure does, but no one is willing to lend the bank money at a lower rate, so based on the original price and the current state (rates rose + the customer wants his money back) the bank will be short those $50 that went to the t-bond seller who can now buy the same bond back for $50 less.