It's not at all clear this violates "Econ 101". Supply and demand set prices, and wages are basically the price for labor. Increasing the supply of labor should necessarily decrease wages, and vice versa.
Imagine the extreme, worst case scenario. If the US opened it's borders completely, and imported every poor worker in the entire world into its borders. Do you really believe that average American worker would be better off? That they could compete with that, and be able to demand a decent wage?
>trade necessarily benefits both sides of the exchange
True, but extremely misleading. E.g. f you move factories overseas for cheaper labor, the local workers lose out, even though other classes may gain. The benefits of trade are not distributed among the entire population.
>cheaper labor => cheaper goods and services
Sure, but if everyone is making proportionally less money, they can't afford the cheaper services, and on net they are poorer.