SAP R/3 : Why cant the valid to date be changed for a role in SU01?

Hi.

I have a few doubts on role assignments / unassignments.

When a role is assigned to a user indirectly based on position, am able to remove the role for any particular user from PO 13. And say change / put a to date of yesterday, and save, the role immediately gets removed from the user.

However when i dont know whether the role was assigned directly or indirectly.

how to find this first whether the user is assigned based on position or the user is assigned roles directly?

And if i want to unassign/remove a particular role from a user, SU01 - does not really allow me to remove.

When i looked at SU10 and put the user name and then say USER/CHange and put the role name - put the to date as yesterday. And click radio button remove. and say save. It flashes some message.

Am not sure whether am looking at things the right way.

Why is that we are unable to change - to date validity of any role from SU01 ?

So that the access to a particular role can be removed immediately for any particular user on SU01.

Can anyone help me understand this.

Thanks

indu