Assuming the reporting isn't highly misleading it seems like this search is the one wired reported the FBI admitting was noncompliant. So this isn't some congressman complaining about a legal search but the FBI violating the law. Whether or not there is a good reason to investigate this congressman or they could have gotten a warrant doesn't matter - they broke the law in this search.
Ok, the FBI says it was a misunderstanding of the rules. Fine, but even the greenest new agent is going to realize combing through a congressman's u minimized communications is a big deal so if they can misunderstand here it means they either don't care or the rules/enforcement mechanism are broken. And that's what the other cases in the rest of the article suggest: a pattern of repeated egregious abuse that is pretty hard to explain away as mere misunderstandings. What exactly did the agents think justified the use to investigate people who applied to join the citizen's academy?
True, the more recent reforms give elected officials more protection but that's small comfort for the rest of us. In my opinion, they not only should need to seek a warrant for accessing the unminimized data but there should be a security cleared lawyer whose job is to oppose the warrant.