Why did he establish judges?
'Rashi': When the Navi told him that eradicating the Asheros was a merit, he checked the Sefer Torah to see if he did properly. Initially, he saw written "Abed Te'abdun...; va'Ashereihem Tisrefun ba'Esh", and thought that this excludes Asheros of Yisrael 1 . He later saw in Parshas Shofetim "v'Lo Sita Lecha Asherah", and rejoiced that he did properly. He also saw there "Shoterim v'Shofetim Titen Lecha", and immediately stationed established judges in every city of Yehudah.
Malbim: It was to judge matters Bein Adam l'Chavero.
If so, why did he eradicate them? Also, why would the Torah command not to eradicate them?! Perhaps the verse teaches to burn only Nochrim's Asheiros, but a Yisrael's should be eradicated in another way. However, 'Rashi' implies unlike this. (PF)
Did he put judges only in fortified cities?
'Rashi': First he put established judges in fortified cities to forcibly judge people. He later put judges not so established in unfortified cities, for people there feared to refuse [to obey] more than those who dwell in the fortified cities, who trusted in their fortified cities.
