Inappropriate screenshot

"Games are accessible only to those who own them, while the community is accessible to all of Steam's users." Is responsability of a parent what the child buys with the credit card but you are resonsable of posting the capture. Pretty self-explanatory. Sorry for your loss anyway.

Steam runs a business and so they allow developers to add/remove patches to make it available for everyone if the content is too mature. Prime example is Nekopara which originally has H scenes but to make it available for community the developers removed the inappropriate content. The users apply the same patch again which makes the H scenes visible again which Steam can't do anything about except to remove screenshots as you said. It is a small price to pay for the benefit which both Steam and the developers can get. This is what I understand.

I guess think about it like...

okay, have you ever been in a gas station, and they have nudie magazines behind mostly-opaque plastic on the top rack in the back, out of reach and hidden from children?

There's a difference between you picking up the magazine, buying it and going home, and you picking up the magazine, opening it up, and displaying the NSFW content openly in the store for anyone to see if they look in your direction. In the second case, the management would probably ask you to not do that.

It's not a 1:1 analogue but I can see where they're coming from. Also consider nudity laws are different in different countries, and Valve has to be concerned with that.

Your argument is very faulty. Whether or not a game is available for anyone to buy is irrelevant. It seems that you only have in mind horny teens who you think would like to get their hands on some soft porn anyways. What about people who strictly want to avoid certain types of imagery? People can buy anything on Steam, but most people will buy only content that they want to see, and by taking out sensitive content and making it visible to all your friends in the activity feed, as well as the wider Steam community, you are likely to offend, shock or hurt people who never signed up for porn or violence.

Doesn't a game with an explicit content require the user to fill in his date of birth before accessing the store page?

Maybe they could offer an option of marking your screenshot as such as well, requiring filling date of birth to access it as well.

I think it's more likely that Steam doesn't care but they do not want to draw the negative attention of groups that do. By censoring what is publicly available they significantly decrease their chance of becoming a target. Another possibility is that this is done to avoid being added to a net nanny censorship database which would almost certainly negatively affect sales.

Its about being Legal. When a young kid goes to look at the Witcher 3 game it asks for them to put in their birthday. Steam is not at fault if the kid enters a fake date to see the content. When a kid goes to your profile though, there is no age verification, so the kiddie can see that nude screenshot you took to "save" for "later." It is hard for them to put an age verification on user content so if someone reports it then they take it down. I guess they could just add a thing where you can flag your stuff as mature so people have to go through age verification to get to your stuff. Steam doesn't really care, but i'm sure Steam lawyers care as showing mature content to kiddies wouldn't go well for them in court.