They use VigLink to add affiliate tags to the external links of the sites that use them.
For Amazon at least, intercepting and tagging URLs is in violation of the Associates agreement, and if detected Amazon will not pay these fraudulent commission claims.
If the URLs are being tagged with the Associates account of the web page owner, then this auto-tagging is a CMS feature, which is reasonable.
If the URLs being tagged are content created by NOT the website owners (like, say, forum posts), then we might be back in fraud territory.
This is actually an optional service (Outbound Links) that can be turned on or off (opt-in by default). No affiliate links are added without turning the feature on.
Some mangled affiliate links cover the cost for 10th largest traffic site nowadays?
Referring someone through an Amazon link doesn't just credit you if they purchase that product you linked to - it credits you for any purchases they make during the cookied period. And for the whole amount of the checkout.
A couple months ago I made over $150 with that same amount of traffic because someone bought a MacBook Air (~$50 in commission), some fancy espresso maker (~$20) and a bunch of other things. Amazon has mastered the art of the upsell.