Its like that for a reason. Since Windows XP is a multi-user environment, one Windows user account has 1 OE6 identity. If another person wants a new identity, create a Windows user account, login as that account, and go into OE6. Presto.
Many of our customers use OE for the simplicity in switching from one ID to another within one PC login - otherwise they would opt for Outlook which is on the vast majority of the syss. Also, OE only locks on a portion ( a good portion, but not a majority) of the systems. That tells me it's not so much a design issue as a glitch. Of course, it could also depend on sys configurations, which we don't have.