I think a big part of the history which people are forgetting here is that there were certain actions which Microsoft engaged in that strengthened the determination of "monopolistic" behavior. Namely, the restraints and difficulty they created for other operating systems in the market. [1][2]
This specific instance ignores that particular history, and focuses just on the "browser included, supported, and otherwise enabled with the most features" part of the argument, which by its self does not seem to stand on its own merits. And, in fact, there really is -no- difference here between Apple and Microsoft - both have created a specific version of their software for a specific hardware platform, and both have placed, or are placing restrictions on what software can run on those platforms with the stated goal of controlling the end-customer experience.
There is not yet any evidence of undue influence on the part of Microsoft to prevent, or otherwise make too costly for, other operating systems, with other browsers, to be installed on ARM-based hardware sold in the market place. In fact, at this point, it seems to be exactly the opposite - that all existing ARM-based tablet style devices currently for sale in the market contain non-Microsoft operating systems, and very few ARM-based phone style devices contain a Microsoft operating system. One would do well to remember that it was the sum of all actions which led to the anti-trust case, not just preventing IE from being uninstalled effectively, or preventing Netscape from being installed at the same level and with the same capabilities as IE.
In this case, there is no evidence that MS is taking other actions to prevent people from selling arm-based devices with other operating systems. Given that we accept that it is legal for other entities to restrict what 3rd party software may be installed on their devices so long as they don't apply undue influence on the market outside of their product, I don't see why the reasoning would be so different here.
[1] http://en.wikipedia.org/wiki/United_States_v._Microsoft
"... Microsoft's conduct in forming restrictive licensing agreements with original equipment manufacturer (OEMs), and Microsoft's intent in its course of conduct. "
[2] http://www.justice.gov/opa/pr/Pre_96/July94/94387.txt.html
"Exclusionary Per Processor Licenses--Microsoft makes its
MS-DOS and Windows technology available on a "per processor"
basis, which requires PC manufacturers to pay a fee to Microsoft for each computer shipped, whether or not the computer contains Microsoft operating system software. The complaint alleges that this arrangement gives Microsoft an unfair advantage by causing a manufacturer selling a non-Microsoft operating system to pay at least two royalties--one to Microsoft and one to its competitor--
thereby making a non-Microsoft unit more expensive. "Microsoft has used its monopoly power, in effect, to levy a
"tax" on PC manufacturers who would otherwise like to offer an alternative system," said Bingaman. "As a result, the ability of rival operating systems to compete has been impeded, innovation has been slowed and consumer choices have been limited.""