In the Cupertino company’s own words:

“Apple requires apps with user generated content to effectively moderate to ensure objectionable, potentially harmful content is filtered out. Content that threatens the wellbeing of others or is intended to incite violence or other lawless acts has never been acceptable on the ‌App Store‌. [Parler] appears to continue to be used to plan and facilitate yet further illegal and dangerous activities […] in violation of Guideline 1.1 – Safety – Objectionable Content.”

Source: Why Did Apple Ban Parler, And Should Investors Care? – Apple Maven

(….just like Twitter and Facebook.   Apple should ban them too. Totally bogus.)