For Americans, it could be as simple as "un-American".
As this sort of accusation is rather rare, I'll cite some ACTUAL examples:
Exhibit 1: Skepticism is "anti-American"
“Have you tried to sabotage as hard as you’ve been doing with (SCHEME NAME REDACTED) to those real ponzi that are running from offshore? I’ve found you’re an anti-american, a traitor and maybe a crook that takes advantage of the American legal system. It seems to me that you make a living by sabotaging American companies. Do you understand what a global economy is? People like you are the one who are giving away American jobs to undesirable countries and in the process benefit from it.”For your information, this suspect scheme is believed to operate both as a Ponzi scheme AND a pyramid scheme hybrid, claims to have 1.2 million affiliates, and pays up to 1.5% DAILY ROI if you put in some money, but they insist they are NOT an investment.
Apparently, believing "if it's too good to be true, it probably is", is anti-American and traitorous, to people defending a suspect scheme.