There's a disconnect here in the comments I'm seeing. The plaintiffs likely were not sophisticated on a level that they're reasonably expected the understand portfolio margin. To the extent that Bob Morse (who seems to have a solid grasp of the concept...at the very least) feels he should qualify himself as not an expert, I think we can take the plaintiffs' lack of understanding for granted.
Secondly, they were under the direction and advice of a financial advisor.
So, it's petty clear they did not understand the risk that took on and placed a reasonable degree of trust in someone who was adequately qualified (weather qualified or not, I have little doubt this conforms to the plaintiffs' beliefs). So they shouldn't be left holding the bag, especially considering that a third party (IBKR) fully aware of and familiar with these risks was engaged in their behalf to provide services, and provides these services as a business for profit.
The advisor may well have had a reasonable belief that this was a temporary condition and wait and see was an appropriate response to these conditions. Prior experience with IBKR may have confirmed this.
IBKR took these actions against the plaintiffs (who signed the broker agreement under the advice of their investment advisor), not the broker.
And finally, no one has directly addressed if the instruments subject to portfolio margin treatment are enumerated by law (if they are and ETNs are not included, this is clearly the plaintiffs' win).
Now, let's assume the advisor lacks sufficient insurance or assets to compensate the plaintiffs for their loss. We're left with who should foot the bill for this? IBKR, who has the resources and with the sophistication of a publicly traded company specifically engaged in operations to turn profit on this risk; or demonstrably unsophisticated investors for whom this is a life altering financial hit?
Two pockets. You're on the jury. Do you ruin someone's life, or do you dent a company's profit over what we might consider a courtesy call after they unilaterally altered the requirements as prescribed by previously agreed contract (which may constitute illusory contact language, and thus unenforceable).
Who do you award the verdict to?