It's actually very simple:

MIT/BSD licenses are pro-business - any business can take the product, change a few lines and redistribute the result without making their changes available.

GPL is pro-user - anyone who gets the source, makes changes, and then redistributes the result has to make their changed sources available as well.

The FSF has written extensively on why (in their opinion) you should prefer copyleft licenses over non-copyleft licenses, but they don't require a license to be copyleft in order to be considered free. It's worth spending a bit of time on their site to understand their point of view. Just be careful not to drink too much of the Kool-Aid or you'll become one of those annoying people who never shut up about the GPL on forums.