In reality GOG isn't going to avoid selling such games because there is far too much money to be had by selling them, and it is up to the publishers whether or not they want to include pre-order DLC or not. If a publisher wants to incentivize people with pre-order DLC and GOG says "we don't allow that" then the publisher will simply not sell their game on GOG and GOG is out any new-release profit that they would get from such a game, and the profit they would have received does not go into enhancing the website or investing in other ways to bring new content to the platform.

By not having such a policy it potentially opens up the number of games they have available to them instead of Steam or some other store making all of the profit off of the given game exclusively, while it lets the GOG customer decide whether they are ok with it or not instead of GOG making that decision unilaterally for all customers themselves. This way customers that do not like pre-order DLC can - like you - boycott such games themselves and simply not buy it, but customers that don't have a problem with it can go ahead and buy it without having to go to Steam, and get a DRM-free game.

Possibilities:

GOG doesn't sell games with pre-order DLC:
- GOG customers who are indifferent about it can not buy the game at all
- GOG customers who want the pre-order DLC can't buy it on GOG either
- GOG customers who loathe pre-order DLC don't have to buy it

GOG does sell games with pre-order DLC:
- GOG customers who are indifferent about it can buy the game on GOG
- GOG customers who want the pre-order DLC can buy it on GOG
- GOG customers who loathe pre-order DLC don't have to buy it

In both cases, people who loathe or simply do not want pre-order DLC do not have to buy it, so that is common to both choices. The only people who are affected at all, are the people who are either indifferent towards it or specifically want it. In the first case they don't have any choice because GOG would have made that choice for them, so they now have to go buy it somewhere else. In the second case where GOG does sell games and lets the customer decide, the people who are indifferent or favour it have the ability to buy it here at GOG if they prefer to do so. So option #2 - GOG selling the stuff is the option that gives customers the largest amount of personal choice in the matter rather than GOG deciding for them.

Having said all of that, I personally am not a fan of pre-order DLC and think it is a carrot on a stick marketing ploy that can be pretty shitty depending on what the bonus stuff actually is. But I am a fan of people being able to decide upon these things for themselves as an individual rather than a small group of people who hate something demanding that it be banned simply because they don't like it when they're not forced to buy it in the first place, and thus removing the rights of others to buy it who might be ok with it.

So I dislike the idea, and can get what I want by simply not buying things that I disagree with, without interfering with what someone else likes. Plus, I'd rather see GOG able to sell such games if there are people who do want them and aren't bothered by this because this makes GOG more profitable and the more profitable they are the more money they have for marketing, going to conferences and showing great sales figures to big game publishers to draw various other games here that GOG customers want to have. Also it adds more insulation against GOG ending up like Desura, Shinyloot, Dotemu or other now defunct stores. These indirect benefits are good for all GOG customers in the end because we end up with more choices.

Sometimes the best thing we can do if we don't like something is to just not buy it, rather than to try to prevent everyone else from buying it too.