The reason they implemented it that way is because Google did not yet provide APIs to facilitate contact import. As Google adopted more secure standards like OAuth, LinkedIn started using the official GMail API features like "import contacts," rather than logging into your account on your behalf.
People underestimate just how far privacy/security have come since 2013 (pre-Snowden), when even major websites still used HTTP on their payment portals. Someone could sit in a coffeeshop with FireSheep and alter your Amazon order. Privacy enhancing features like OAuth, TLS, and 2FA have only become widespread in the last 7-8 years.