Don't kid yourself. If I go to England and break their laws, I expect to be tried by British law, not American.
Not a battle worth stiring up without a darn good reason...
To me the reason is obvious.
Yes, you would be tried under their laws, but you have a right to speak to a U.S. consulate, and they are supposed to inform you of that right.
I assure you that if you found out 50 americans were about to be put to death and were in mexican prisons, having never been granted consultation with the American embassy, people here would be up in arms.
Last time this came up, someone showed me that this was not a signed treaty, if so then probably the courts won’t allow enforcement of it.