There is a very practical reason for choosing a copyleft license as an author. If you choose a non-copyleft free software license, then downstream users actually have more options than you do. They can take your code, add small modifications and then release the result under a non-free license. Even if you implement those features as well, you aren't playing on a level playing field. You have to support all of the software and they only have to support their small additions. The end result is that they can offer a product with a value-add at a lower cost than you can. You are essentially competing with yourself.

Of course you can decide not to release your code under a free software license, but that rather defeats the purpose of running a free software project ;-)

Looking at it another way, more permissive licences grant more options to downstream users than copyleft licenses. It doesn't make sense to argue that you are offering only copyleft licenses to benefit those downstream users individually (and I know of nobody who makes that claim). Instead, the argument is that it is better for the group that everybody has equal restrictions and can't use proprietary code to gain an advantage over everyone else.

This is actually one of the reasons why if anyone asks me to assign copyright for my side projects over for work, I'm happy to do so: on the provision that all of my work is licensed under the GPL. It barely matters to me at that point who owns the copyright (though to protect yourself further you should ensure that no one person has copyright over the entire work).

IMHO, although the choice to write free software at all is often ideological, the choice of using a copyleft license or not is usually pragmatic -- at least among those who understand why copyleft licenses are written the way they are.