well the usgov don't have jurisdiction to do whatever they like wherever they like in the world
No, but they do have jurisdiction over the US market. Just like a Chinese OEM cant sell RF equipment on the US market if it doesnt comply with FCC regulation, or illegal drugs for that matter, you cant sell securities in the US market that dont comply with SEC regulation. That the purchase happens over the internet doesnt change a thing. If you dont like SEC requirements (which frankly are for the most part quite sane disclosure requirements that apply to almost any civilized country), sell your securities elsewhere, which is what bitfunder is doing by blocking US investors.
BTW, while SEC regulations might not be perfectly suited for the kind of securities we see on bitcoin exchanges, personally I have no problems whatsoever with forcing registration of these securities, giving us at the very least basic disclosure of the people, company and business plans behind them. It would prevent at least 95% of all the scams, make it much more easy to invest wisely, while costing next to nothing.
Now legally operating an exchange under current laws would be another matter, and probably out of reach of bitfunder & Co, but even there I will not shed a tear to see them go and get replaced with much more trustworthy companies that do have the resources to operate legally, like Second Market. Its not like we were hurt to see the early bitcoin currency exchanges get replaced or regulated, I would love to see a similar move to more professionalism for security exchanges so we dont depend on amateurs (/scammers) running toy exchanges like GLBSE, Bitcoinica, Mpex, picostocks, and yeah, bitfunder. As it is, there is no snow balls chance in hell we could ever attract serious investment money and we are left gambling with our own playmoney on 50% scam securities on exchanges that disappear every 6 months, usually along with their anonymous operator.