IANAL, but the product tying interpretation of US antitrust law specifies that making one product or service a mandatory addition to the purchase of a different product or service can be considered an anticompetitive practice.

The "feature" in question in their application was "searching the web". Imagine if they shipped a default e-mail client that only allowed you to use Gmail, and you had to install a separate client to use e-mail with any other e-mail provider. How many people are going to just use Gmail rather than go find some other app and provider? That's an unfair advantage due to product tying.

Microsoft bundled IE into Windows 98 in a way that you could not remove. Microsoft argued that, hey, it's just a standard part of an operating system, and you can get something else, what's the big deal. The government didn't buy it. They settled before the government could reach a final ruling about whether this was anticompetitive. https://en.wikipedia.org/wiki/Tying_(commerce)#Microsoft_pro...