Interesting question, and one we discussed a lot when designing the system - the winning reason was, that we scan the url prior to rendering it anyway, and there's nothing to stop a site which we think is Green today from becoming Red in the future. What's the harm in allowing the creation of a link to a known bad site? I guess it's only dangerous if somehow a route around our frame and banner is discovered.
What do you think? Do you think we should forbid creation of links to known bad sites? Some of my engineers said they would like to be able to use the short link to share URLs of evidence with other researchers - it would block them, but that's a very limited use case.
open to thoughts on this for sure...
PS- and I forgot to add, checking the link at time of creation will slow things down - we keep a cache of used links so once one person has clicked a short link, it should be really fast for everyone else, but of course on creation we have to do a live lookup. Something to think about.