Also with this being fanmade and all Symmetra could technically at some point seen the error of her ways,  I like to view it that way at least after this comic:
http://www.funnyjunk.com/channel/overwatch-time/Lucio+and+symmetra/krDkLxe#8e4293_5973812

Other then that it's up to the OP on why Symmetra decided to align herself with a reformed Overwatch.