They don't have to offer the same salary, just salary in the same range. That range can be pretty wide ($20k+)

Employees on an H1-B visa have drastically less job mobility than US Citizens. This creates a power advantage for the employer.

>but this is Google

Google has, in the past, illegally conspired to prevent other companies from recruiting their employees. This lowers wages and reduces employee mobility. Clearly there's incentive because they have literally broken the law in the past to achieve these results.