I had this exact conversation with some friends yesterday, and here was my response to them edited for clarity:

And the difference between Parler and Signal is that Parler is a social media platform, Signal is a messaging platform. It's the difference of having a group of terrorists[0] in your restaurant everyday audibly planning a terrorist attack versus being the site of a message drop that occurs discreetly. Accessory charges are likely in the former unlikely in the latter.[1]

[0]Replace terrorists with criminals or whatever is palatable to you, it was the example we were using at the time.

[1]Standard disclosure I am not a lawyer/prosecutor/etc.