And because it's the character. That has always been integral part of Steph in her appearances, both in the good and in the bad. She doesn't think before jumping in or doing things, which at times result in epic disasters, such as causing a gang war in Gotham for two times in a row. She hasn't been shown to rise above that or do pretty much anything to redeem herself for the thing that happened, except now apparantely she decided to put on the Batsymbol without seeking any kind of approval, which is hell of a different thing than to just wearing her own outfit.
Besides, everything happens because that's what the writers decide to write. You can't just choose to stick with every time they portray the character in the positive light and complain when it is in the negative light, especially with any character even tangentially involved with Batclan.