Bitcoins on the other hand, don't have value on their own, and if nobody wants to accept them in exchange for dollars, goods or services, no legal action can be taken by their 'owners' to get some kind of value out of them. As such, Bitcons are just empty numbers on the internet, which means that people are exchanging real values for literally nothing.
That's all well and good, but fiat suffers from the same problem. They're just paper without demand, and you just can't magically say banks will bail you out. If your currency fails, you'd best keep in mind that you're not the only client they have to appease. There have been precedent for that, too -- currencies have failed before, the Zimbabwean Dollar being one of the most recent examples. Bitcoins have no intrinsic value, and neither does fiat.
As for legal support, well, decentralization comes with certain liberties, but at the same time, you have to let go of some privileges. We all know we're completely responsible for ourselves. One thing is for sure though: we're not trading nothing. Value is a man-made concept, and for as long as there is demand for Bitcoins, it will never be just numbers.