OnLoad Javascript Creating Write Priv Errors for Read-Only Users

All,

Can someone explain to me if javascript for onLoad functions that write to an entity run under user context?

I have a few read-only users that get privWriteLead errors when opening up a lead.  Left me a bit confused, but remember I have js that writes to Lead onLoad.  I got rid of the error by giving the user read-write license level.

1)Can someone give me a low level explanation on why this happened?

2)How can I modify code to let this code write to entity for read-only user? -- I am confident this was root cause

Thanks and Regards.