|The behavior we have now is more useful, actually. It's there for people to narrow down their search. Imagine instead, for example, you're searching for "solo". You're immediately bombarded by a whole image dump of solo images of Moroboshi Kirari, who you absolutely hate. You can click the minus button to remove images of her from your search. At that point, you have a nice, clean search clear of all of the character that you don't want to see.|
If you're proposing that this change only take effect for tags that are already in your search bar, that creates an issue of buttons that look identical doing different things that can be very confusing to users who don't know what they do. We're explicitly trying not to have a confusing user interface. It's poor interface design to make a button do something different without making the button look different.