For ordinary participants, HN means a fun conversation, inspiring articles, maybe a life-changing job offer or FOSS collaboration for a small percentage of those who comment, already a small proportion of HN's readers. Y Combinator effectively runs HN on a shoestring for a small amount of public relations influence, and I would guess that HN's existence doesn't even make the agenda at YC board meetings.
With LinkedIn, every post and association is directly linked to your professional image. You can't view LinkedIn posts without an account, so passive consumption of interesting articles simply cannot happen by design. Microsoft runs LinkedIn to legally* extract vast amounts of personal information in order to exert influence on job markets for their paying customers, and they clearly thought the control was worth purchasing for US$2620000000.
The point I would like to make is that although the forced positivity isn't a fundamentally different dynamic from HN's forced intellectualism, almost everyone on HN voluntarily participates, but opting-out of LinkedIn's positivity proletariat can have dire consequences for one's livelihood.
* where law is defined as 'might is right'