TheOptimizationIdiot - 9:14 pm on May 7, 2013 (gmt 0)
It's browsers following w3.org protocol (web standards).
Tweet a link to an HTTP site of yours and click the link. Then go check and see if it's recorded by your stat program as a Twitter referral. Twitter didn't "stop sending referrer data". Compliant browsers don't send a referrer when a link is clicked on an HTTPS page and points to an HTTP page. Yes, Google could circumvent protocol and make associations in Analytics if they wanted to badly enough, but that's way more "conspiracy theory" than just following protocol.
And, although the data may look the same for AdWords:
1.) For Google.com search clicks: The search was conducted on Google.com (secure connection). The searcher knowingly/willingly sent the search to Google.com (secure connection). The Ad was displayed on Google.com (secure connection). The click was recorded on Google.com (secure connection). The Ad clicked is recorded via secure connection prior to the visitor being sent to the landing page. That click is associated with the landing page set for the account.
2.) For other sites that are HTTP: the information can be sent according to protocol, because it's information sent from an HTTP connection to an HTTPS connection or another HTTP connection. (Sending from "less secure" (HTTP) to "more secure" (HTTPS) is allowed. Sending HTTPS to HTTPS is allowed too. Sending HTTP to HTTP is even allowed. Sending HTTPS to HTTP is not.) For HTTPS sites: the information can be sent to Google, because the site the information is from and the site receiving the information are both HTTPS.
* Also: If Google really decided they didn't want to send referrer data all they have to do is add rel="noreferrer" to their links and no HTML5 compliant browser would send it, so they could totally quit easily if they wanted to, but they haven't done that yet to my knowledge.