That said, I don't think it is entirely fair to pile on Oracle for this problem. Oracle doesn't seem especially atypical of the rest of the industry or even the bottom of the barrel. I saw much worse treatment at my first US employer for myself and others. Mid-tier employers and body-shop consultancy companies use underpaying people as a strategic core component of their profitability.
I had prospective employers directly say that they were pleased they could use my immigration status to pay me less, specifically that they would offer employer sponsored permanent residency application instead of paying me competitive rates, bonuses, promotions.
If you want to end the abuses decouple visas and permanent resident applications from the employer or make it much easier to maintain or continue status and applications when moving between employers.