It was a different situation. Microsoft Windows was considered a monopoly, with around 95% of the desktop market (there was no mobile market yet). Microsoft used this to gain a monopoly in desktop browsers. When you are a monopoly, other rules apply, you cannot just abuse your customers and competitors with it.

They absolutely did. The whole antitrust case revolved around Microsoft threatening OEMs to terminate their reseller agreements if they sold the machines with Netscape Navigator pre-installed. It is the literal definition of monopoly abuse!

Pardon me for not being more precise -- they never prevented end users from installing Netscape on their machines.

If we assume for the sake of argument that a particular customer would knowingly buy a computer from an OEM who pre-installed Netscape on it, then the OEM is the user of the computer before the end user buys it. Why should Microsoft be allowed to prohibit such a pre-installation? For security, maybe, but that probably wasn't Microsoft's motivation.

By that logic Apple does not prevent end users from rooting the phone and installing anything they like, it just makes it very inconvenient... and less shady than Microsoft's shell execute bug.

How is shipping OS with preinstalled apps shitty?

Makes it much harder for any similar apps to compete, even if they're better. So you end up with worse overall outcomes.