They did not send the requests INTENTIONALLY. LinkledIn has a feature where if you let it, it scans your associated contact list (gmail, yahoo, hotmail) and tries to match them up with their database and make an introduction for you. If no association, you get the emails that have caused you you heartache. It's one of those sneaky little things that if you don't actively opt out of something, you are implicitly giving them your blessings (like those old 8 cd's for a penny from the 90's). This is probably what your friends did and don't know it. Whenever linkedin or facebook accounts want to sync your contacts, red flag!!
3 years ago I managed to contact them and asked them to blacklist my email. I haven't got any spam anymore.