email - despite its legendary years of service - is still very dependent on an email client to be able to display and manage it.
While avast can detect and isolate part of an email stream that may be subject to infection there is no simple way of allowing the email message to be displayed and managed without the significant risk of exposing the user to whatever virus has been detected in the message itself.
While you or I might be able to manage the infected datastream in a "notepad" environment avast has to be designed for the vast majority of users for whom security is the top priority and where risk of infection must be preferably eliminated or, at least, minimised. For me - I believe that avast has a duty to defer to the needs of the majority and prevent - as far as possible - any risk of infection.