I’m an immigrant myself and know how the visa application process works. There are a few mechanisms in place to prevent H-1B holders to receive lower wages than their colleagues, first and foremost the PERM, which guarantees similar wages in the same occupation and area of employment.
If Google or anyone else are hiring H-1B visa holders directly, they have to pay them accordingly to their own standards.
Also, visa salaries have to be publicly available. It’s not like they could lie about it.
When people speak of visa holders as “cheap” labour, they probably think of IT consultants hired by Indian companies, which regularly are the worst offenders.