Rugby Rule Question

Why is not retreating the 10 metres after a penalty never considered a professional foul and sin bin offence? To me it seems like a blatant professional foul. For example: penalty awarded 10 metres out from a scrum, number 8 quick tap and runs only to be tackled a few metres before the line by another forward who didn't retreat. Ref plays blows for another penalty but doesn't take any further action, when if the forward didn't make the tackle there's a good chance the no 8 would've scored. Do referees ever trea this as a professsional foul? I've watched a fair bit of rugby in my life and can't remember once where not retreating thr 10 has resulted in a sin bin, anyone know why this is?

It's more of an advantage offence. Similar to that of offside. Ref would never give a penalty try if the team has been offside. Penalty tries are only awarded in blatant offence situations and some might argue that not retreating 10 meters is not a blatant offence, as some players are just slow. Or something like that.

I believe refs "interpret it" a little... e.g. if the attacking player runs INTO a defender... or if a defender makes an honest effort

Rugby isn't referred with the same mindset as soccer. If it was, the game would resemble the careful shuffling around the buffet lunch at a hemophilia convention.