Ashington is 100% correct in his question. Bilic justified his actions later by saying "he had the right" to offer the referee his opinion and to ask for justification for the referees' actions. Where?
Premier League and Football League managers have more access to the match officials (directly and indirectly) than at any other level, but this supposed right does not exist. Although the wording, structure and content of Law Five has changed since I took the exam back in 1980-frozen-to-death; it never has said that the referee has a duty to stand there and take dog's abuse (dressed up and polite enquiries) from players, coaches, etc. Nor does it say they have a duty to justify their decisions to platers, coaches, etc.
As for the penalties, the original decision was absurd (bit correct) and the retake was a matter of fact. The fact that many other instances go unpunished does not make the decision incorrect. Try running that argument to a traffic policeman when he is booking you for 35mph in a 30 zone.