LinkedIn actually sued HiQ Labs, which scraped LinkedIn to do exactly this (and this extensions scanning is likely a defense mechanism against similar attacks):

https://epic.org/documents/linkedin-corp-v-hiq-labs-inc/

> HiQ has created two specific data products targeted at employers: (1) “Keeper,” which informs employers which of their employees are at “risk” of being recruited by competitors; and...

My hunch is that HiQ simply looked for spikes in activity on LinkedIn as a signal for a job hunt: https://news.ycombinator.com/item?id=47566893

In any case, this lawsuit was discussed a few times on HN at the time, and IIRC there were a fair bit of support for allowing free scraping of "public information." Interesting how the sentiment here has turned these days...