Why does Big Companies treat employees Bad?

I work for meijer store 231 in Southfield, Michigan. I am 18 and just now graduating high school. I seek to further my education but I wish to do so in another state. I have been working for meijer for about three going on four years. I have performed many jobs such as utility worker, cashier, Gm clerk, and electronic sales member. Even though I have worked for meijer ever since I started high school and never asked for any of my school events off. I have never went to any school activities or any clubs. I work from when school lets out to around 10pm or 11pm at night. I just wanted to know why am I getting such a hard time trying to further my education. My managers at store 231 are insistent on keeping me there and refuse to let me register for school. I have been a loyal worker for meijers all these years and I just wanted to know why am I getting such a hard time. I dont even plan to quit meijers I actually just want to transfer to a store in the state that I am moving too. It seems as though they are just trying to fire me for trying to go to school. Many of my coworkers told me that they always give people who try to go to school a hard time, but why would they do that. If I owned a company I would want intelligent employees. I just don't understand why they would do all this to their employees. I am just one of many who have stated this is a terrible company to work for. I have a friend who's aunt hurt her back working for miejers and they refuse to pay her medical bills. It is a shame they are allowed to treat people this way it is inhumane.