What's wrong with marking content NSFW? Its not censoring it, simply providing a warning to those who may be in a place where their screen is visible and that might not be appropriate.
Right, it's not quite the same because it's the submitter in reddit and the community on Twitter. I'm not trying to agree with the top parent comment about reddit.
Twitter doesn't fully block NSFW content, either. They each have a system requiring a click to unveil or profile setting.
The effect is to obscure the content, so that people just scrolling by won't see it. Someone has to notice it's hidden, trust that it something they do want to see and not a disturbing gore or porn pic, and then click to see it. That has the effect of reducing the reach of the media. It's similar to how people would make messages on the craigslist RnR forum disappear by abusing flagging.
On Reddit, people opposing your message can use voting rings and automation to abuse downvoting, which has the effect of hiding content.
They can complain that your image needs to be NSFW, and ask the moderators to change it. I believe moderators can set it for content, but if you are posting in their sub, you are at their mercy anyway.
People usually don't want to voluntarily censor things from themselves. They want to impose their personal moral or political ideology on everyone else.