It's a winning business strategy, especially when you have legions of die-hard customers who won't leave you. Microsoft has a similar situation (though many probably won't leave MS for different reasons), so it's in their business interest to increase profits by subjugating their customers with annoying ads baked into the OS. What do they have to lose? It's not like Apple's/MS's customers are going to switch to Linux (which has its own problems of course).

To be fair to Apple, it does sound like this is a somewhat rare bug (only seen in high-load situations?). It's not as bad, I think, as ads in the OS. So, as you say, Apple just needs to be a little better than MS and that's good enough.