I dont get it. If someone accepts a deposit and hold fiat currency for you, and they still have it, how the f*** can they legally deny to pay that money which is mine and which they have?

? I dont get it, sorry!
If you were dealing with an actual bank, they couldn't do that.
But MtGox isn't a bank, isn't regulated as a bank, and you have none of the safeguards you would have had if they were a bank.