I'm not sure it's deliberately obscure, but rather a victim of circumstance.
For example, the UX team's requirements might have been:
- We need to show how long ago a post/tweet was shared, as it creates a sense of immediacy and relevance
- We need to provide a link an individual post/tweet
- We need to cut down on UI clutter for the majority of users
- 99.9% of shared posts/tweets impressions won't involve the user clicking the link to an individual post/tweet
- The time/date doesn't have an action, and it has a relevance to the permanent state of that post.
As it is, when you click a post in your Stream in Twitter, it expands and the word "Details" appears - which has the same action as clicking the timestamp. So really, it's in two places in Twitter, but they're both slightly abstracted from the normal reading experience.
In short: they probably found that the function was used so infrequently, and only by power-users, that this solution was better than forcing everyone to see "Permalink" on each post.
Equally, people tend to only want the permanent URL to reshare it, and both systems provide their own Retweet and Share functionalities which, when sharing on that same community, are superior to getting a URL, pasting it into your composition area and writing a personal message.