This is a false dichotomy. The law doesn't see "all comers" as equal; specifically, there are what's called "protected classes" in United States civil rights law. Businesses (and individuals while conducting certain kinds of business) are not allowed to discriminate on certain bases (e.g. race, religion, veteran status), but they are allowed to discriminate on others.
The issue where the wedding bakers' case was interesting was that it pit two values against each other - in one sense it was discrimination against a protected class (sexual orientation can be a protected class, depending on state law and interpretation of federal law), and in another it was compelling speech (again, depending on your interpretation of "speech") that had religious implications.
Reddit, on the other hand, is not acting against a protected class by banning e.g. /r/fatpeoplehate, /r/whiterights, or /r/incels. If the managements' ethics or customer backlash prompts them to, they can ban away. In some cases, if speech might be illegal in some ways (e.g. incitement to violence, harassment, or copyright infringement (yes, the fact that that is on the list is a bit ridiculous)) they are legally obligated to remove it.