He didn't exactly "prop them up". Microsoft bought $150MM shares of non-voting stock (which they sold at a huge profit). Apple didn't need money at the time, they needed legitimacy, which this investment provided.
In return for this, Apple dropped a multitude of patent-infringement lawsuits against Microsoft (which Apple appeared to be wining), and licensed those technologies to Microsoft. These were not related to Microsoft's anti-trust issues, nor was the existence of Apple. It's OK that Microsoft was a monopoly - monopolies are legal - it's the abuse of the power the monopoly gave them.