In the olden days of Wyse and VT-100 terminals and serial connections, before networks and GUIs and emails and iPads were invented, users on UNIX systems (and those old character-based terminals) could send messages to other users with commands like "write" and do broadcasts with "wall". You would use "who" or "users" to see who else was logged into the system and what they were doing and then send a message. You tried not to do this while the other user was editing something or doing something else that your message would screw up, which is why you needed to see what they were doing. More than 25 years later, the current Linux releases still have those commands. The way the commands worked has never changed and users can still see some information about each other.

From a security point of view, it makes little difference. It's not seeing someone else's processes that is the security problem; it's that you got to a shell prompt in the first place. These days, secure UNIX and Linux systems force regular users directly into an application on login and then disconnect them when they exit the app. You NEVER let a regular user get to a shell prompt where they can run things like "ps" and "top" and "users". So, the system you're talking about is intrinsically insecure...

As for privacy, any time you're using a system that someone else owns you are not entitled to any expectation of privacy. All of your files and activities are open for inspection by the system owners or whomever else they allow to look at them. So, if you want a private system, you'll have to build one of your own and then don't let anybody else log into it.

Cheers!