It's because not everyone ... actually the vast majority of users ... is aware of other browsers and even if they would be, they may have then only known about Firefox even though there are many many more browsers.
Think of it this way, it's akin to a car manufacturer selling cars with labels that state that anything other than XYZ branded gasoline will cause damage and void the warranty. Sure, maybe it won't cause damage and sure it may not stand up in court that it voids the warranty, but most people aren't in the position to know that, nor should they need to be in the position to know.
We ... the community of technology savy ... are very frequently remiss to ignore that not everyone can be, nor should have to be well informed about technology. Think of if Martha Stewart claimed that her recipe will only turn out if you use Greenest Pastures Milk when you are trying to hobble together a cake to impress your family. Are you informed enough about the intricacies of milk that you would know that you can use other milk brands?
Sure, you may scoff and say that you know enough about milk to know that it's not true, but do you know that some brands don't have additives or certain types of fat that may collapse your batter?
Your car analogy is poor. Better would be to consider a scenario where Ford owned 95% of the car market, and offered for free, Ford branded stereos in each model whereas previously there was no stereo and you had to buy one separate.
Other stereo manufacturers, who sell stereos got mad, and basically convinced the government that Ford has a monopoly and should be forced to offer consumers a choice in stereo.
Microsoft never warned people that using another browser would "cause damage" or void warranty. They simply gave away something for free, that was previously a paid product. Their dominant market position gave them leverage to essentially wipe out the competition. This, depending on your market philosophy was either the right move to protect consumers, or an example of special interests influencing government to effect markets (IMHO it was probably both).
Think of it this way, it's akin to a car manufacturer selling cars with labels that state that anything other than XYZ branded gasoline will cause damage and void the warranty. Sure, maybe it won't cause damage and sure it may not stand up in court that it voids the warranty, but most people aren't in the position to know that, nor should they need to be in the position to know.
We ... the community of technology savy ... are very frequently remiss to ignore that not everyone can be, nor should have to be well informed about technology. Think of if Martha Stewart claimed that her recipe will only turn out if you use Greenest Pastures Milk when you are trying to hobble together a cake to impress your family. Are you informed enough about the intricacies of milk that you would know that you can use other milk brands?
Sure, you may scoff and say that you know enough about milk to know that it's not true, but do you know that some brands don't have additives or certain types of fat that may collapse your batter?