Obligatory car analogy: Apple's position in tablets and phones is similar to Ford's position in cars. Ford will not sell you a car with a Chevy engine. Ford is in the complete car market, and an engine is an essential part of a complete car.
Antitrust law tries to be reasonable. This means that for those things that are the main or essential components of a produce, the manufacturer should be free to choose those components.
If antitrust law looked too deeply into individual components, costs for everyone would go way up. Imagine the manufacturing nightmare if, say, Dell had to worry that if they used capacitors from Dielectric Laboratories in their motherboards, they might get sued for not also offering the choice of capacitors from Johnson Dielectrics, ROHM Products, or Illinois Capacitor.
For tablets and smartphones, internet browsing is practically one of the defining features of the product. A typical consumer who bought a tablet and found it had no browser preinstalled would be about as shocked as someone who bought a Ford and found no engine installed.