The middle management & line workers of Twitter can monetize their small Twitter stock portfolios pretty quick without shifting the market all that much. Jack, on the other hand, would take a major hit if he tried to sell off his.
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Clarification on content_sesh's good point: I don't assume whether Jack agrees or disagrees with Trump's points. They are billionaires, might be "frenemies" for all we know. I do assume Jack likes a cash-cow for his major property, Twitter - and tries to balance the upsides & downsides of hosting Trump's contentious content.