I have never participated in any change.org campaign for a similar reason: You cannot sign to oppose, so you end up with an extremely distorted view of a given matter.
But that's always how petitions have worked. Whoever created the petition is only interested in collecting signatures of people who agree with their proposal, so that they can bring it to legislators, authorities, etc. and say that 'this many people also want this'. Collecting signatures of people who disagree is counter-productive.
I have never participated in any change.org campaign for a similar reason: You cannot sign to oppose, so you end up with an extremely distorted view of a given matter.