I'm not suggesting the behavior is criminal, only that it is a sign of partisanship which could be leading to biases which is why the language on the Clinton e-mails were softened.
Something doesn't have to be illegal for it to be investigated. Just like depositing a shit ton of money into a bank out of the blue would lead to some red flags going up at a financial institution which could trigger IRS taking a look - all of that is completely legal but it may be a sign of money laundering or tax evasion so an investigation may be warranted.
That all being said, if he did nothing wrong why did Mueller drop him like a hot potato?