There's no doubt that it's causing significant harm now, but I think you're not seeing this from a longer term perspective: if they think continuing to allow political discussion is likely to create more workplace problems in the future, then that could outweigh any problems they cause now in their hiring pipeline. Two possible points in favor of this trade-off:

1. I've heard vague hearsay that it's hard to get a job there and they don't hire that often, they're not a big company with a big revolving door. Hiring new employees may be a less frequent and less consequential problem than avoiding workplace issues that affect existing employees.

2. This has apparently enhanced the company's image amongst some people, just read over this HN thread. It's not clear that this will damage their hiring appeal and overall ability to competently fill positions.