I don't remember where I heard this rule, maybe it was UA; it sounds something like: "If we were reasonable to believe you would be denied entry, and we were wrong, you still don't get compensated for IDB." This essentially means "We're not responsible for sincere errors."

If translated this to the OP's situation though, "If you have another visa we'll let you board," and "Let's ask the USCBP guy" have completely invalidated their claim to any "sincere error"!