I think they got away with that either because it was before rules, its not technically in a historical district, or
It was built in 1983 and I guess they never thought it was going to be a historical district, with One Riverwalk, and all the glass at the IBC and Whitacre Tower facing the river and being built after that.
With the HDRC in place, you would think that downtown would look more like this.
or this:
found those and a bunch of old postcards
here.