That legal threat is already there. AFAK, all the securities being traded there are unregistered, and offered to US public. Thats illegal under US law (almost certainly the same applies to EU and pretty much everywhere else), there are no 2 ways about it.
The securities potentially being scams, I dont really see what extra legal threat that may incur for anyone other than the issuers.
Also, the idea that an exchange should be responsible for weeding out scams is ludicrous. An exchange doesnt have the incentive (scams make them money too), doesnt have the authority, probably doesnt have the know how. It would just create a false sense of security.
I would however, encourage exchanges to require asset issuers to prove their identity so at the very least legal action can be taken if things go sour.