Although perhaps I'm misunderstanding and the idea is that each wallet device requires a separate implementation and this is just Apple's version? I guess that makes sense. But if the product is "easily make passes for your business" and then they are Apple-locked, that is obnoxious and anti-consumer and people should refuse to use it.
Apple Pass Designer is just a tool for customizing what the passes a business is giving to its iOS customers looks like. It's macOS-only (Apple barely supports anything not-Apple). I assume the customizations won't translate to other apps unless those apps add support, but the passes should still work.