Cops and federal agents often paint a one sided portrait of the reason why they want to kick down a door or search someone's computer. And yes, the cops already believe that the target of the warrant has done something criminal.
Search warrants are often based upon tips provided by "past proven and reliable" snitches (drug addicts, thieves, hookers). Hence, law requires the cops have to perform some level of corroboration -- i.e., a controlled buy from the target's house.
Because FISA warrants deal with national security, the affidavits are presented to a FISA Court panel. The affiant is sworn under oath and is subject to questions from the bench (grill session). When it comes to suspected terrorists, do you think the FBI includes all possible exculpatory information that suggest the target is NOT a terrorist??
As to Carter Page, four different FISA Court judges signed off on warrants.
What cops or agents cannot do is fabricate facts or omit material facts that would otherwise change the probable cause calculus.
Was the FBI less than forthcoming re the Carter Page FISA warrants in question? Doubtful, but if so, would the omission(s) (if any) have altered the probable cause determination by the FISA Court? I would need to see the four corners of the affidavits in question, and review the underlying intelligence.
We know that Nunes never even read the FISA warrants that his memo criticizes. That, by itself, is all one needs to know about the memo's credibility.
None of the above has anything to do with the Mueller investigation.