Restricting private "webmail" access from government employer network?

["evb" <swiver () cox net>]

It seems some employers decide to block access to personal email/webmail
from employer networks, but then later change their mind, in favor of
allowing private (non-work related) webmail access.  

Having researched the issue a bit, supposedly there are very few valid
reasons to allow private email/webmail access from a corporate network, and
many reasons in favor of restriction/banning.

If this is true, why do employers nonetheless change their mind mind in
favor of allowing webmail access?  What is/are the deciding factors?

Thx,

Eric