Twitter puts a nofollow attribute on its links to reduce incentive for spammers to tweet. Nofollow means search indexers don't crawl from that link nor count it toward any ranking. (Twitter would have low SEO values anyway because it has too high a concentration of links compared to text.)

As for short URLs, they shouldn't be designed to steal page rank. If they do a proper 301 redirect, then the original address should receive the SEO value.