This is probably why Apple doesn't expose WebKit as a public API on tvOS: regardless of how useful it would be, the danger is too great that you'll end up with garbage, lowest-common-denomiator web apps littering your platform.

That said, I think the OP has a point that JS could have been used instead of BrightScript--no DOM, no possibility to re-use a generic or non-native UX design, just using the language. Given the timing though, it's hard to fault the choices made 10+ years ago.