Most "pizza places" will not hire someone full-time. They want part time employees so that they do not have to deal with benefits.
Now, the intelligent thing to do as a nation would be to offload those benefits to the government so that companies would be free of the extra work AND employees would be free to move to better jobs... but America is not the land of the smart.
They want part time employees because it offers scheduling flexibility. If all you have is full time workers and someone calls out sick, you're short a person. If you have mostly part time workers, you can call everyone not scheduled that day to see if they can come in. Office jobs don't really have this because it's typically not critical that the employee is there.
In your rush to defend the actions of asshole bosses, you miss an important part: people who are trying to make a living with part time jobs are going to get another part time job. Not only are those people NOT more flexible than full time employees, but half the time they are less available than full time workers because they are stuck on a treadmill where not being at both jobs equals a loss in hours for the next week. There are tons of full time hourly employees who will JUMP at overtime.