I agree with those who say that sales of Homefront will matter more than the reviews.
If you promise quality and deliver mediocrity, the market should punish you. That hasn't happened to CoD because its sales have been stellar.
Franchises are powerful, and I have no doubt that CoD's strength as a franchise has helped it - so far. However, its single-player was - according to reviews - very well paced, with many climactic moments and modes of play to break up the monotony of take cover, shoot, repeat. According to Russ Pitts, pacing was Homefront's major failure; it was simply more of the same all the way through, and nothing ever felt like it really mattered; you were simply grabbing fuel trucks for the resistance - contrast to CoD, where you're retaking the White House.
As for the score, I agree that 73 should be "good," but in the language of the industry, it is mediocre. That's the way it is. Hell, percentages in school work the same way; you should be used to it by now.