It is very easy to understand. LinkedIn started as a service to help with business networking but ended up just being a recruiter place and scamming (yes, scamming) people making them think the service has many other uses.

For example, you go to a LinkedIn group about marketing with million of subscribers, post an average interesting non spammy article and nobody comments or click on the links. Nobody! And it has million users. Compare this to posting in an interesting subreddit like /r/sysadmin or /r/networking or /r/python where almost anonymous people give you an answer to a complex issue.

Then you decide to go premium, or pay for ads where you will find that the conversion is practically zero. Finally you apply to their API. Yes, you apply because they manually review your use case and will not approve your request even if you just want to retrieve all your connections with a simple HTTP GET.