Answer: Because they believe that Dr. Sheldon Cooper was not teaching to they're liking, but Penny and Howard are different because Penny was serious about wanting to learn, where as Howard purposefully disrespected Sheldon.
Character analysis:
Penny was careless and disregarded taking note as she should have, as well as, was not listening to what Sheldon had to say, instead, demanding a summery on the topic of physics. As well as being disruptive to her professor's time.
Howard disrespected out of anger and played music on his phone all to get back at his professor and his actions of trying to make him look less intelligent then he was. On top of that, he spat at the instructor with out