I'm going to start by saying I really don't know, but here are my guesses. It's all about the way they collect data for advertising and for analytics.

For example, Twitter gives you the option to tailor itself (its ads?) based on the websites you visit. http://i.imgur.com/EWU2l.png Facebook tracks you with their Like button, I imagine Twitter is doing something similar. And while they can track you while logged out, they would probably be better off if they could tie that information to all the followers, followees, and behaviors associated with an account.

My second example is embedded tweets. Although I can't find the article or blog post, I remember seeing a while back that Twitter wanted to start pushing people to use their embedded tweets rather than images. https://dev.twitter.com/docs/embedded-tweets

Aside from providing the user with a better experience (like being able to click on the users' names to go to their profiles) or letting Twitter control the user experience, this also allows Twitter to embed tracking pixels. Highlighted in this picture are the tracking pixels loaded in Chrome when I embedded the latest tweet from the Twitter account on a web page. http://i.imgur.com/SeVaQ.png

And this next example wasn't made for the web, but it plays into how they're trying to boost their own analaytics. While wrapping every link with their t.co url shortener does give them control over thing like killing spam links, it also lets them track which links became popular from which tweets. That's why they'll still use their t.co links on already short links (reminder: in my opinion). I believe this solution works out for them because if someone clicks a link from their official Android app or from a 3rd party app like MetroTwit, they can still track the referral came from a tweet.

To summarize, I believe they're trying to boost their advertising relevance and trying to improve their analytics offerings to business accounts by using browser cookies.

So if I understand you correctly, you're saying that the spite Gruber mentions comes from the fact that they can't track you I'm the native app. That's a perfectly reasonable assumption but I don't think it really has much to do with spite. I read in another comment that maybe they're getting rid of all traces of the old guard's legacy and that seems to me more likely to be where the spite Gruber mentions plays into this.

Oh, sorry. I misread "In any case, I'm actually really interested in the comment about spite Gruber makes at the end." as "In any case, I'm actually really interested in the comment Gruber makes at the end. Can anyone answer that?"

So I interpreted the comment at the end as "They want people to use the website."