So look, I'm summarizing why lynching Joseph is good:
1. He's easily the scummiest player around. Just reread if you don't believe him. Textbook scum tells. Now you may say "it's too obvious", but this would be Joseph's first scum play, and because of that, it's reasonable that he would make these mistakes.
2. Everyone has taken a stand. Lynching Joseph will give us a lot of data to analyze - how did people act towards the lynch? We get much more out of this than say, out of lynching a lurker.
3. The 1-shot Cop is useless. If Joseph is indeed town, then maybe scum will kill him tonight to prevent an investigation. What's much more likely though is that they will simply roleblock him. In that case we don't get any information, and our Cop shot goes to waste. Much better to give the shot to the unknown UB, who has in fact a chance of actually catching scum with it.