They can, but let's not pretend that Parler won't be targeted for destruction somehow if it gets a critical mass of users. We've seen this strategy employed before on right-wing platforms that catch too much negative media attention. Their infrastructure platforms will be targeted, their distribution channels will be targeted, their payment systems will be targeted, their code licenses will be targeted, the media will try to dig up whatever negative stories about the firm or its owners that they can, etc. Eventually some platform will be able to resist this but it's an ugly process.