They weren't just found guilty of shipping a browser in their OS. They were found guilty of using their OS dominant/monopoly position to have unfair advantages in multiple areas (browsers, media players, and servers). It's all connected. The ballot screen was seen as a possible way to undo some of the damage from the anticompetitive move. The EU specifically cites case T-201/04 and IP/04/382 to establish Windows as a dominant OS. The complaint from Sun is what kicked off IP/04/382. Without it, the browser action would have not happened in the EU.

The US cases were also connected in the same way. The cases against Microsoft's forcing of OEMs to buy Windows for every PC out the door then helped the larger anti trust case kick off.

This is the key reason Ubuntu, RedHat, Be, Apple, or any other OS vendor has not seen similar action from the EU regarding browsers in the OS. They lack a dominant/monopoly position, so they don't fall under anti trust laws the same way. Both the US and EU have these additional anti trust protections that only come into the picture when a monopoly exists.

Having a monopoly is not illegal, but using it to quash competition in non monopoly markets is. How that monopoly is obtained isn't relevant. It could be via previously ruled illegal actions like Windows, or reaching that position via legitimate competitive actions in a once crowded market. The end state is all that comes into play when being judged to have a monopoly position or not.

All of this is key to both cases Google faces. Has Google used it's dominant position it legitimately earned in search to then have an unfair advantage in other web services? (The EU in the linked article at the top specifically cites Google Shopping). This has nothing to do with competition directly in the search space with Bing and others.

Does Google have a dominant position in the mobile OS space, and if so has it used that position to gain unfair advantages in other areas?

Skyhook here remains a key example for me on the Android side. Skyhook had a lucrative business being able to provide more accurate location tracking via WiFi detection. Skyhook and a few companies who shipped Android devices entered into contracts to ship their technology. Google forced those companies to shred those contracts, or face the risk of not shipping their devices with Android. And in turn later moved all location services out of the open source part of Android and all into the closed off Google Play Services.

Similar to how Microsoft went from having IE as a separate download to a very integrated piece of the OS, and forcing OEMs to shred contracts with Netscape. An action by Microsoft that would have had far less impact had they not had dominated the OS space.