Presidents didn't have lifetime secret service protection, and if I recall VP ended as soon as the term ended.
You have example of accommodations related to work, or when somebody testifies against dangerous criminal. I don't recall we provide any assistance when somebody is going to jail to ensure equality. In fact the only thing we do makes things unequal.
Reality Winner was deemed a flight risk when she leaked a single document to newspaper. This guy was free to walk immediately after, didn't even have to pay a bail.
As for risk of assassination. The secret service is provided by the government. So is jail. The government makes sure that convicts don't escape but also controls who gets in. To protect inmate from others they can isolate them as well.
How come? He is no longer a president. He also broke the law. It sounds like no laws should apply to him anymore.
Also, shouldn't we also provide Secret Service for people like Pelosi or Schiff? They weren't even indicted and there are people who want to murder them.