I remember flying in the copilot seat with a laptop on my lap in a six seater. I think this was... four weeks ago? Didn't seem to compromise any kind of safety. Do you really think if someone wanted to electronically tamper with the plane they would be as obvious as opening a laptop and working on an email?
Even if it's harmless, I don't think it's his place to make a decision about whether electronic interference is dangerous to the plane's equipment or not and affect the safety of everyone else on board. The steward on the other hand is completely justified in enforcing/following the airline's (industry?) policy.