on LinkedIn, twitter couldn't control the inventory, so they couldn't monetize the content.

It was their tradeoff of reducing usage while users are on LI, while hopefully increasing consumption on their own site from those users.

It's certainly a gamble, since I'd argue that LI was more a source of new twitter users rather than siphoning off views from already loyal twitter users.

The other possible threat in Twitter's head might have been LI bootstrapping their newsfeed off Twitter (which they'd mostly been doing) and then growing it large enough to actively compete with Twitter.