Arena Trap was banned because the ability itself is unhealthy, even on weaker users like Trapinch or Diglett. On their own they may not be dominant forces, but they can still enable very oppressive dynamics -> para-spam is already a big thing in DPP and Trapinch could make really good use of it, restricting counterplays to setup mons like CM Clefable, which was already extremely oppressive alongside Dugtrio. Similarly, Diglett has seen use in BW OU after Dugtrio’s ban, and it would likely show up in DPP as well. In short, every mon with Arena Trap ends up being a nuisance, so the ability as a whole was banned.
Dynamic Punch is different. Many Pokémon can learn it, but only Machamp makes truly problematic use of it. That’s why Machamp is the one banned, not the move itself.