Out of curiosity, I just went through the list of mega pokemon and out of all of them, there are only 24/74 (less than a third of the megas) that didn't increase both offenses, and that's specifically being generous and including dragonite and zygarde, who lower their main attacking stat to boost the one they originally didn't use.
And then only a third of those (alakazam, gengar, mewtwo x, heracross, houndoom, mawile, aggron, and manectric) were in the first batch of 30 megas introduced in X and Y (26.66%). So it is actually a tradition to waste stats, yeah.