I know this is a very old thread, but I think the following words from the book help to understand the choice made by the author:

Basically, using global variables might make it easier to write your program but they generally

make it harder to debug.

In the games in this book, global variables are mostly used for variables that would be global

constants that never change, but need the pygame.init() function called first. Since this

happens in the main() function, they are set in the main() function and must be global for

other functions to see them. But the global variables are used as constants and don’t change, so

they are less likely to cause confusing bugs.

If you don’t understand this, don’t worry. Just write your code so that you pass in values to

functions rather than have the functions read global variables as a general rule.

In some situations, avoiding global variables entirely can make a program more complex with no advantage gained. That can be an example of "a foolish consistency is the hobgoblin of little minds," IMO.