Basically aesthetics. Especially with amazon, if I'm on mobile and copy a link (https://www.amazon.com/Haribo-Gummi-Candy-Gold-Bears-5-Pound...), it's got a bunch of excess weight. I can just cut it down to https://www.amazon.com/dp/B000EVOSE4/ without any loss of data. Alternatively, you can use bit.ly or an extension (https://chrome.google.com/webstore/detail/native-url-shorten...) to shorten it to http://amzn.to/2aTrW7v. However, this is a pain on mobile. The share buttons should just work. I think they might do it this way for increased analytics or referral links, however they should be able to do that with bit.ly too.