As we know, John doesn't seem to think that he has to put in much effort to be a good father. To his way of thinking, he's provided his children with a roof over their heads so he thinks that that should be enough. This means that time and again, he sees having to be a father when it's inconvenient as a cruel imposition willed on him by children who ask too much because they're selfish and spoiled. When he looks at fathers who support their children in their endeavors while letting their own interests slide, he sees not good men behaving like good men, he sees suckers being exploited by brats. What is supposed to happen is that children speak seldom and ask little of their father because he works all day.
This belief that Daddy needs his quiet so his children owe it to him to be grinning little dummies out of fawning gratitude is complicated by something that has enraged a lot of us: his hateful belief that disagreeing with him in the slightest is not a sign that someone is an individual with a right to his or her own opinion but a defiant brat who needs to be either threatened with violence, shipped away like a criminal or arbitrarily spanked so as to be reminded that there can be only one opinion: John's. What makes him even more intolerable is not that he's a thug who hates to be disagreed with; it's that he's a thug who isn't honest with himself. Always and ever, he acts like the helpless people he dominates are trying to destroy him, as if they're a viable threat. This means that 'straight-shooter' is Lynnglish for "cowardly asshole."
Another fact to be considered is that, as this excerpt from one of his letters:
Apparently Mike is getting close to finishing his novel. I always hated writing essays, and this project of his seems like one giant essay to me, so I can't imagine how he could either want to do it, or more importantly enjoy the writing! He seems to be enjoying it, though, in spite of all the chaos with his young family.
indicates, John cannot bring himself to care about any interests that do not interest him. Simply put, John can't relate to being a writer because he hates having to ask himself why he does what he does so Mike's career is the subject of dismissive laughter because Daddy is a knob who doesn't like to stretch his mental horizons. Since he shares Elly's dislike of music owing to sharing her being an embittered failure who resents those with the will to dare and to do (as well as a need to hide this from herself), it doesn't take a genius to see why he wants to toss the instrument of torturing him by making have to interact with his family on their terms under a steamroller.