I do not understand some of that, if properly run only BAT scripts is what purpose you can select other types of PS1, VBS, JS ?
I think initially it's been implemented that way to ease selection of appropriate files. Say, for pre/post-job activity you can easily select a needed ps script and, then, just add Powershell.exe at the beginning of the command to make everything work properly. (see here)
Most likely, that dialog form has been added to pre-freeze/post-thaw script window without changes. However, those scripts being injected inserted inside guests are quite different and the behaviour described about doesn't work any longer and can even lead to unintended consequences.
We do understand the confusion caused by that and plan to address it in the next product release by modifying that dialog.