The sites aren’t linking to a tweet, they’re embedding them. There’s a difference of intent between a plain old <a> link to a tweet’s URL, and the full set of tags, scripts, and configuration used to embed a tweet inline with your page. It would be unreasonable to hold you accountable for a simple link if I had a browser plugin that automatically converted them into embeds, but if you used the twitter markup such that they would be rendered as embeds on any standards compliant browser that’s a different story.