Clearly, Amuchastegui got to the semis fair/square. A decent leg turk, a good singlet grab here/there to keep your opponent on his back, ...and you're in. Deserving/earned.
Weird that Ruth's left leg was injured, but his right leg took all of the abuse as the trapped leg. It was crazy. Until the 3rd period, still thought he would pull a rabbit of the hat. Credit to Amuchastegui on the turns, but Ruth was literally tied down - or 'strapped' down, if you will.
I'd call that impeding Ruth's path to the 4. That's a foul.
Grabbing headgear = okay (*ref: see Suriano vs. Fix). Grabbing singlets = not okay (unless uncalled).