I think the way Hornady made it headspace shorter is safer than if they made it longer:
If somebody takes 6mm ARC factory ammunition and tries to fire it in a 6mm Grendel rifle (a reasonable possibility for someone ignorant) - it chambers fine, the bolt can close and lock up properly - helping prevent an out-of-battery discharge - and it may or may not fire due to the probable light primer strike. If it fires, you get case stretch and at worst the case ruptures, gas is diverted back through the action and leaves part of the case in the chamber as the case head is extracted. Most of the time this should result in no injury to the shooter, as most rifle systems are designed to safely handle this kind of case failure.
If Hornady made the 6 ARC headspace longer than the 6 Grendel, it could still chamber, but the bolt would not lock up properly, and in a poorly functioning rifle system, that could result in an out of battery discharge - which is a devastating failure.
I don't think the potential case of a handloader taking their 6mm Grendel handloads and attempting to fire them in a 6 ARC rifle (creating a potential out-of-battery discharge) is a reasonable safety concern for any company. That is squarely in the personal responsibility category when you are fabricating your own non-standardized ammunition. It's a use case that doesn't involve both factory rifle systems and ammunition combinations, which would be cause for liability.
How is the 6 ARC any less safe than for any other factory cartridge offering?