Anti-trust law is not simple. In the case of MSFT and IE they were accused under the protects of the Sherman Anti-Trust Act[1] of using their monopoly position (Windows) to reduce consumer choice (bundling IE without letting users pick a different option). In the case of Apple and Google, it's more likely to fall under the Clayton Antitrust Act[2] as it's a matter of corporations collaborating to reduce consumer choice (but I am not a lawyer, and not even American, so I could be waaaay off the mark here.)
> That only applies when you dealing with markets with an absence of competition.
This is incorrect. From the FTC[1]:
> Courts do not require a literal monopoly before applying rules for single firm conduct; that term is used as shorthand for a firm with significant and durable market power — that is, the long term ability to raise price or exclude competitors. That is how that term is used here: a "monopolist" is a firm with significant and durable market power.
For example, Microsoft had ~95% market share of the entire computing market when they tried to embrace and extinguish Netscape.
It's why it was perfectly fine for Google to use its web services to influence competition within the Android OS market.