What's particularly insidious about a lot of these link shorteners is the use of non-semantic redirects. That is, redirects which are not based on HTTP Location: headers but things like meta http-equiv="Refresh". I assume this is done to allow these pages to be loaded with tracking scripts.

Of course this is a completely broken way to implement a link shortener since it won't work with non-browser tools such as curl. I tried a t.co URL with curl and it returns a Location: header, which means they're doing user agent sniffing. If you need to use user agent sniffing to make something practical, it's generally a good sign you shouldn't be doing that thing.