I guess I'm struggling quite to see the use case here. For web-site providers themselves they would much rather do the conditional logic themselves because then that would apply to all visitors not just those that come via your shorturl service.

I can perhaps imagine someone tweeting something like: "I've just published a new book, get it here", where 'here' is a link, which might send the user to the correct Amazon page based on their location (eg. amazon.com for US users and amazon.co.uk for UK based users).