Anyone wanting to bypass the 140 limit is already doing so using an image with text in it. And anyone who spams really long messages using URLs is going to see their followers count drop pretty rapidly.

Hasn't twitter been replacing long URLs with their own short URLs for a while now? So this change probably won't make a difference, assuming users will still have to hover the link to see the tooltip of the full URL.

The short answer is that they won't care much.

Twitter's job right now is to figure out how to make their UX less niche and grow market share before they lose ground to the Whatsapps and Snapchats of the world, not to police edge cases that are better managed by simple social norms.