An offshored worker is already much cheaper than an H1B worker, I would expect any easy substitutions along those lines to already be performed. Probably some effect on the margins, but I would doubt it outweighs the primary effect.

(Of course, it would be a problem if you think H1Bs are for hiring people who cannot be found domestically, but it does not seem like many people think that these days.)

By penalizing offshoring. I don't say this as a particularly nationalistic person either. All companies in all countries should be heavily incentivized to hire local labor and sell to the local market. Globalization is extremely beneficial of course but the various side effects need to be managed.