Monopolies are not illegal under antitrust. What is illegal are certain practices designed to lead to monopoly, including using dominance in one market to try to gain power in another, unrelated market.
By bundling the browser and penalizing manufacturers who wanted to bundle a different browser in addition or instead of Microsoft's, Microsoft was using their dominance in one market (desktop operating systems for x86 PCs) to try to gain power in an unrelated market (internet browsing applications that run on Windows). Hence, they ran into antitrust problems.
In tablets and smartphones, there has not really been a separate market for internet browsing applications. The market for tablets and smartphones has been for complete internet devices (hardware, OS, and browser) from the start.
By bundling the browser and penalizing manufacturers who wanted to bundle a different browser in addition or instead of Microsoft's, Microsoft was using their dominance in one market (desktop operating systems for x86 PCs) to try to gain power in an unrelated market (internet browsing applications that run on Windows). Hence, they ran into antitrust problems.
In tablets and smartphones, there has not really been a separate market for internet browsing applications. The market for tablets and smartphones has been for complete internet devices (hardware, OS, and browser) from the start.