You're getting a bit confused here. Gedmatch cooperates with law enforcement but it's only if you've chosen to, so it's a program you need to opt in to. This is legal.
Some of what you've found is about how the police use DNA in general, for example going into bins to get you or your relatives DNA, this is unrelated to genetic genealogy and has been done for decades.
Now one thing that could happen is police requesting your DNA by court order, this is already done, not through genetic genealogy though, they can just get it from you. If the police get a court order to obtain your DNA then they're swabbing you themselves, or as previously mentioned, just getting it from your bin.
Police can not request everyones DNA by court order. That's not how laws work, and if they wanted to use genetic genealogy privately then they'd need access to the entire database, millions of people in dozens of countries, and each one would need to be requested individually with a full case to obtain. That's impossible.
Police do have their own database of DNA they've legitimately obtained, it's called CODIS. This can be used to find close relatives, so if your brother was arrested for a robbery and had his DNA collected, then your DNA was found in a murder scene they could link it to your brother using CODIS.