autosandbox in 'auto' mode is not meant for users to decide if the program is malicious or not. in 'auto' mode, avast does the analysis for the users and will provide the end result of either, 'we did not find enough evidence to identify the file as malware', or, 'the file appears to be malware'.
source (for end result analysis of avast):
https://blog.avast.com/2012/03/20/autosandbox-why-are-you-annoying-me/my guess of the reason for auto terminating is to prevent average users from using the program in the sandbox. this is because any work done in programs running within sandbox will not be saved - potentially wasting the users time and effort.
if u want a more 'hands-on approach', u can change the autosandbox mode to 'ask'. that way, programs running within the autosandbox will not be terminated. in this mode, there is no visible analysis from avast.
my guess of the reason for not auto terminating in 'ask' mode is that if u know how to change the autosandbox settings from 'auto' to 'ask', u are more likely to know that any work done with programs in sandbox will not be saved - catering to more advanced users?
that said this need confirmation from avast team