Not all theaters were equipment to show widescreen in mid and late 1950s and probably well into 1960s (This is what "be glad what you got crowd" seems to ignore at all cost). So the matting presentation cannot be justified. Too further underline that many pre 1953 shorts were also reissued to theaters well into 1950 and 1960s.
While I'm somewhat in agreement that not all of the post-widescreen era WB cartoons are blocked for 1.85-1 (although the titles most certainly are), I must point out that the facts do not jive with the above. In a survey that was taken in December, 1953 by BoxOffice
magazine, it was shown that 58% of theaters in the U.S. were running widescreen in some form or another. By September, 1956, this number was estimated to be in the 90-95% range. Many, many industrial photos show interiors of theaters in even the smallest towns made a conversion during that era.
For what it's worth, I've run many original 35mm prints of the WB and other series to study this. While WB's practice is questionable, nearly every other studio was blocking their animation for either 1.66 (early Paramount WS cartoons), 1.75 (early MGM WS cartoons), or 1.85-1 (everything after about 1954).