That's a feature that has been supported for years, and while it's interesting that you're only just seeing the "share" link hitting the site now, the behaviour is somewhat required.
The alternative to you, the sender, loading the page is the recipient loading the page. As you're the one who has chosen the page to share it seems reasonable that you should be the one taking the "leak the page load to the site" hit, and you send the recipient a nice message packaging up the preview image and text and when they receive it they can choose whether or not to actually contact that server.
To answer "why not just reuse the loaded content": the title on the page may have been influenced by JS, specific login, etc and in addition sites may want to serve different content for a preview (maybe a headline image or whatever) and they may require additional queries. For example http://waffles.dog includes an image when sent as a message rather than just text saying "It's Waffles!" which is meaningless without context.