If you pass around raw Tweets then there is no way for Twitter to argue that a deleted Tweet is deleted. If instead you force people to recrawl them each time then Tweets can be deleted at source and all subsequent access requests will not return that deleted Tweet.
Personally I think this way of distributing Tweets in bulk is not scalable and acts as a barrier to research. Additionally one could argue that preventing people from having access to static Tweet corpora undermines doing reproducible research.
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