This is true if you hire from third world. I am from Northern Europe, and having a visa revoked and going back to my home country would not be a threat to me. I'd just happily move back home, and find a job there.
There are very few people who wouldn't consider being forced to move across continents, while giving up their rights, a highly biasing factor in the pros/cons of any decision they made.
PG's point is that a lot of people from outside the US (including from Europe) currently would prefer relocating to the US but they have difficulty getting a visa. Long term more and more people would prefer the second best choice that's available to them.
No, from my reading of pg's point: Most of the top programmers live outside of the US, so we should make it easier to get them by changing immigration policy.
This is a good point. But finding out that your employer is so nasty that they use your visa status as a bargaining chip, can be even more inconvenient.
The question of the talent of the new employees is secondary when you realize they will have to move countries in order to be tied to a specific job on penalty of getting their visa revoked.
So suddenly, you create an unlimited class of employees with no bargaining power, and can play them against your native employees. This would be bad enough, but H1B visa holders usually make 20% less than their equivalent American coworkers.