There's a well-known standard of "probable cause". A cop is supposed to know what that standard is, it's part of their job description, and it pertains directly to what they do every day.
When a cop is asked to claim that there's probable cause when said standard isn't met, it's clearly and plainly an illegal order, because they're asked to lie. Doing so under penalty of perjury is doubly illegal. He doesn't need to be a constitutional lawyer to determine that this is illegal and cannot be obeyed.
This is exactly the same as doing a search without warrant, or arresting someone without reading them their Miranda rights, or lying under oath (something that the cop had also admitted to doing, by the way).
That this also happens to be unconstitutional is coincidental. It suffices for the cop to know it's illegal, and act accordingly.
And if they do know their orders are illegal, but follow them anyway, they should be held responsible accordingly.