What's different here is presumably the scale the policy is being applied at, which creates the likelihood that significant groups of employees will be able to demonstrate disparate impact. For it to be a problem, there need to be enough remote employees in "favored" and "disfavored" {location,compensation} buckets to make a case. Gitlab is presumably not there yet.
USG pays flat rate globally.
They then plus up a COLA allowance based on high-cost areas, special circumstances, etc. But the base rate is a flat rate for a given position.
You can pay 'less extra', but not 'less', generally. Which I'm sure is what FB and others will technically do.