(1) If you consider single digits market share to be significant.
There are good reasons for this very simple rule that everyone but Microsoft seems able to comply with. By definition, such a rule cannot be "arbitrary".
If you pride yourself and market yourself on openness any rule that flies in the face of said openness is entirely arbitrary, because there shouldn't be any rules limiting your openness.
It's also the only significant phone OS that doesn't ship with Google search as the default.
How is this any different than me releasing my code under something like the LGPL, which would allow those folks who dynamically link to not release source to their app, but force those who are required to statically link (due to platform or other reasons) to release source to their app?
Note that it's a similar situation. If I release code under the LGPL, i don't have to abide by it, only others do.
Twitter requires third party API users limit themselves to 100k users (or whatever it is), but, even though they use the same API, does not require it of itself.
They also have decided some folks don't have to abide by this.
How is this different?
None of this is "anti-competitive" behavior.
Would you think it anti-competitive behavior if Google provided no API and also told people finding ways to embed it to stop?