Treaties have the full force of law in the US. It makes sense to be wary of them when they are all about domestic policy, just as it makes sense to consider carefully the implications of a constitutional amendment that would impact domestic policy.
Treaties serve a useful purpose for relations between nations and respecting human rights in conflict situations. In this case, the countries that really need to respect the rights of people within their borders are just going to sign it and ignore it, as we've seen with the Convention on the Rights of the Child. Meanwhile, the US is capable on its own to make laws protecting the disabled, apart from any treaty.
^^^ This
Plus I wouldn't trust the UN to operate a lemonade stand.
