What browsers would that include and how would Microsoft work with those behind them to make this possible? These are some possible concerns, but there are other less obvious issues, such as technical hurdles that warrant investigation. Apparently, part of the EU’s demands may require that certain components of IE be disabled if someone should choose a third party browser. It's likely that they mean some of the IE integration, such as it being the preferred application for certain types of content regardless of what browsers are installed.
It is Microsoft, not the EU, who is releasing this information, which makes you wonder what their angle is. Is the company trying to get public support for “IE preservation” in Windows?