Because they're providing an executable which interacts with additional services, and they're based in the US where lawsuits is the default way to settle disagreements.

For example, the point about them getting "royalty-free, unrestricted access to your inputs" is almost certainly to protect against frivolous lawsuits along the lines of "I typed something in the Search Bar and Mozilla copied it and that violated my copyright and so I want $1 billion".

Again, you're free to make your own fork, in which case you're not bound by these terms.