Political ideology is not protected, regardless of how important you perceive basic accommodations like shelter, food, or communication are.
This feels to me like a situation of "fine for anyone to do, problematic for everyone to do". With political ideology not a protected class in many states, anyone is free to refuse service to an outspoken Democratic voter, but if a whole town did that it would be unthinkable. The specious answer of "just make your own business" wouldn't be practical in that scenario (are you going to start your own grocery, gas station, etc?).
To be clear, I'm not trying to make a defense of Parler; my understanding having never signed up is that it was an unmoderated cesspool.I am, however, trying to point out that the end result of individuals making totally reasonable and rational decisions within their rights can produce outcomes that most people would view as wrong.
Now consider that this exact scenario is currently affecting 23.5 million people in the US:
When everyone avoids those areas, though, bam: food desert.
I honestly don't know the right balance to strike here. On the one hand, I fervently believe that businesses shouldn't be forced to act against their interests. On the other hand, though, the sum of these individual actions produces an unacceptable outcome.
I'm sure there's some balance of regulation, incentives, and public-private partnerships that would strike a good balance here, but both the extreme solutions (Groceries can locate wherever they want vs. groceries have to establish branches in food deserts) seem to be totally unsatisfactory, at least in my mind.