The newspaper is Imgflip as a whole: a private publisher of content. Or, you could analogize the newspaper to individual streams on Imgflip with their own mods and their own points of view.
As a private publisher, the newspaper has the absolute right to determine what kinds of articles, advertisements, etc. get run under its name. You can think of all sorts of good reasons why this must and should be: the newspaper has an intended audience (e.g. general community members), the newspaper has a reputation to protect, the newspaper has a specific point of view (e.g. centrist, liberal, etc.)
Bottom-line: The newspaper can deny the pro-Hitler ad. If instead the government had the ability to *force* the newspaper to run the pro-Hitler ad, then this would not be a free-speech country anymore.
As it turns out, anyone seeking to run a pro-Hitler ad can do so if they find a willing publisher, or if they finance and publish it themselves. But the publisher MUST be willing. Fascists can't spew their pro-Nazi crap anywhere and everywhere, as often as they want and to anyone they want, under the guise of "free speech." That's not how it works.
Caption this Meme 