Does the FBI even consider bitcoin as something that actually has any value that requires investigating an accusation of theft?

Market cap isn't money, its an estimate of value based on a premise that is not merely flawed but outright insane -- that every unit of an asset that exists can be liquidated at the current market price of a single unit (that is, that market clearing price is not effected by supply.)

Its used because its easily calculated, and people are prone to want to believe that values that are easy to calculate are also meaningful.