This rule changed so recently that it hasn't trickled into the job market yet.
https://www.uscis.gov/news/news-releases/uscis-publishes-fin...
Effective from Jan 17th, 2017.
Previously, if your employer found out that you interviewed elsewhere (due to to a reference call or other means), they could fire you and there was only 10 days where you could wait for the new employer/offer to materialize.
https://www.uscis.gov/tools/ombudsman-liaison/practical-immi...
So, yes - you're somewhat married to your company.