Law Enforcement Agencies work in cooperation with DOJ and they obtain evidence with the use of warrant[1] which is issued by court.
Speaking of obtaining illegal evidence you can search for numerous court cases where judges turned blind eye on questionable methods of obtaining evidence using hacking techniques. Some of those cases were seizure of Dark Web websites and their servers. Courts will always protect government agencies because they want to be cohesive and in synergy.
But the point being that it’s not universal that judges will facilitate such methods.
[0] https://en.wikipedia.org/wiki/Fruit_of_the_poisonous_tree
"reports on the web say" can often be used to get court ordered investigations - no matter the validity of the anonymous info.
So "illegally obtained info" can spawn "legal" routes to investigate the same...