I think there is a "I know it when I see it" type of argument to say, we will make the strongest effort possible to never censor anything that is not attempting to sell a product but we reserve the right to force advertisements to go through a specific channel.

For better or worse the types Of things which get called to be banned from a platform for being unacceptable speech rarely resemble the specific styles of advertising we classify as spam.

Another argument is let users be ther own spam control.have some idiot acquaintance from middle school who keeps spamming pyramid scheme and/or nazism? Fine, just unfriend them.

As long as you are given control over what content is pushed at you directly, there is no need to sensor the whole platform because of a specific undesirable.