The problem is that the COVID-19 vaccines are not like the vaccines for polio, yellow fever, etc.... Those vaccines are proven and have been around for DECADES.
The current COVID-19 vaccines have been around less than a year and is a much higher risk compared to proven and time tested vaccines.
Right now OSHA has decided that companies are liable if someone becomes injured from a vaccination due to it being required for continued employment. That's why DL said it only new hires are being required to get it because it would open up the company to too much liability if they required their employees to receive it.