We're not "implying" that the rules "state" anything. We're saying that the rules can be interpreted as meaning that, just as your targeting roll = the control roll, something that allows you to control the spell = targeting.
The only place it refers to them as "uncontrolled shifts" and part of backlash is where it says, "Any uncontrolled power taken as backlash remains a part of the spell" which doesn't mean they're still uncontrolled shifts, it means that you're taking backlash because they were uncontrolled--and our thinking is that by taking backlash, those shifts are, in fact, back under your control. They're part of the spell, ergo, they're controlled. If they were out of control, they'd be fallout.
The RAW doesn't come out and say, or provide an example showing, that the base Discipline/targeting roll is unaffected by backlash either. If it did, the discussion wouldn't have started in the first place.
Yes, in your example, it's a good buy, because the base attack roll is pretty good already. If he rolls worse than that, however, it's a wasted spell and the wizard is given a choice between letting the power go as fallout and potentially harming his allies, or "controlling" the power for the sake of a spell that he already is pretty sure is going to be a waste.
This doesn't quite jive with the book's assertion that "if he chooses to absorb it all himself, his spell should still go off as intended because he was willing to pay the extra cost." I imagine it's pretty rare that "miss everybody" is what the wizard intends with an attack spell.
Though the Fallout example in the book could certainly be read to support the backlash = targeting stance. In it, Harry rolls bad enough that his control/targeting roll is only a +2, for a Weapon:5 spell against a whole zone. It then says that Jim B. decides to take it as Fallout because he's taken consequences and his stress tracks are filling up--but says nothing about how a Fair attack roll is unlikely to hit anything. One would think that if that was a factor--and that he'd be stuck with the Fair even if he did take backlash--the example would have mentioned it. This omission could be taken to imply that if Harry had taken it as backlash, it would have remained a worthwhile attack both in power and in accuracy, rather than one that makes a big boom but doesn't hit anything.
Though that brings up another question I had: Say in the Fallout example, Harry rolls a solid +7, and gets his Weapon:5 attack against the zone. Is the targeting roll that the targets have to beat 5 or 7?