I have always thought it was their way of knowing you at least had access to money when needed.
Since ( as of now) they still do not require health insurance it is not a bad idea for them to require a deposit.
The only surprising thing actually is as other have said you only need to have it deposited for a couple of months.
I have long since thought that someday health insurance will be required & since many do not qualify due to preexisting, age, etc etc That they may one day require those without insurance to leave that deposit in place year round as a backup should emergency medical be required.
Would at least cut down on the " Go Fund Me" insurance policies