the reason the from address is blank is because in the PST the from address is stored as an exchange address (something like /o=MyOrg/ou=MySite/cn=Recipients/cn=JoeSmith). The exchange address book provider can be used to resolve these adresses to SMTP addresses. When you import a PST, there is no exchange address book provider available in the profile; its only available in exchange profiles.
To exemplify this, if you open the pst with outlook (in a non-exchange profile) and try to reply to one of those messages, the TO field in the reply message will be blank for the same reason.
If you use the PST import wizard to migrate messages, you will get empty from addresses when the FROM address type is an exchange address. This doesn't happen with the exchange migration wizard because it is able to resolve the addresses to SMTP addresses using the exchange address book provider.