"payments to phone makers to make Google Search the default"
- Not sure how this hurts, as such, as surely other people could pay those makers to be the default? So long as it's changeable? Is this any different to Apple being paid to direct people to Google? That could hurt if that went to court based on this case.
Corporation undertakes behaviour A, perfectly legal.
Corporation becomes a monopoly, same behaviour A is now no longer legal.
When you become a monopoly, many behaviours that you may have engaged in previously, or that other corporations currently engage in, become illegal for you to engage in now, but those other non-monopolies can continue to engage in those behaviours now forbidden to you.
So as a monopoly provider of phone operating systems, it is illegal for Google to pay handset makers to make Google search their default search. However, since say Microsoft, Yahoo and DuckDuckGo are not monopoly phone operating system providers, and also are not monopoly search providers, it would be perfectly legal for them to pay handset providers to set their search engines as default.
I'm not sure why people seem to have difficulty with this concept as it's brought up frequently whenever a company gets fined for anti-trust for doing what seems reasonable or that other companies are doing. It's perfectly straightforward: Once you become a monopoly the rules change, your behaviours become more constrained than when you aren't a monopoly.