This is a property of the internet and information in general. If data was served to clients at some point, then it's possible for the clients to have saved a copy (if nothing else then by taking a picture of the screen). Even if it happens that no one saved a copy, it's not generally possible to be sure of that. Any "deletion" feature would therefore be unreliable at best.
Being able to "edit" posts, to submit a new version of a post and have clients by convention normally show the newest version, sounds like a decent feature. (The older versions would, of course, be available too.)
If you want to post offensive material to some people/governments you need to work hard to stay anonymous, like the reporter example where they recommend using a public eg library place to connect.
What happens then if I want to delete something I posted previously?
Is it free speech if I want to retract something posted by myself but I'm not able to do so?
Is it really free speech if a comment can't be altered/deleted by the owner of it?