It's in the interests of every site owner to prevent "backlink exploits". Any user-submitted content (unless perhaps if it is heavily moderated or submitted by trusted/senior members) is "untrusted". If content is untrusted then any outbound links (if allowed at all) need to be nofollow. This should be standard practise for any site.
The "redirect script" can prevent the link passing "PageRank" since the script can potentially filter out bot traffic. The script can also be blocked by
robots.txt, so any "good" bots won't crawl it anyway. This script can potentially prevent the bot from even seeing the destination of the link.
But as @Stephen pointed out in comments, you don't need to use a script to prevent the mainstream search engine bots using the link to pass "PageRank". A
rel="nofollow" attribute on the anchor is sufficient to prevent PageRank being passed. However, bots can still "see" the link and potentially crawl it (although Google reportedly does not).
A "redirect script" can also allow the site to more easily track the link and gather user stats on those that click it.
However, Google (and probably other search engines) reportedly don't allow links on social media networks to directly affect ranking anyway. So, social media links don't pass "PageRank" by default. Having said that, Google would seem to be able to measure social engagement to some degree and this may play an indirect role in ranking.