The little training that education students get in college concerning classroom management is often either useless or bad. This is probably because the professors are often many years removed from the classroom, if they were ever there in the first place.
"Be mean and lighten up later" seems to be the general theme of the advice given to future teachers for handling classroom management.
Practicality does not seem to be a major goal of college education training.
Figure out a classroom management style that fits your own personality. Have boundaries and consequences, but don't feel like you have to be cranky all the time to be successful. There is more than one way to succeed in the classroom.
Be careful about assuming your professors always knew what they were talking about. This should be true about all subjects in college, but especially so in education.