They could justify it to a court, because "Facebook incubated the insurrection" or "Facebook broke our rules" not because "we don't like enabling our competitors".
https://www.theverge.com/2019/1/31/18206020/apple-facebook-i...
If the violations were serious enough, I think Apple/Google would consider some kind of temporary shutdown for the Facebook app if they thought they could weather the PR storm.
On the other hand, Facebook is not really a competitor to Apple, while Google very much is, so Apple has an incentive not to carry out an action that would strengthen Google.
There is only one logical conclusion and that is that Apple and Google will eventually boot Facebook off their platforms due to increasing amounts of risk associated with mobs and things that allow them to form.