What matters is that his actions can be construed -- rightly or wrongly -- by those of weak or no faith, to mean he is treating Islam as co-equal or legitimate. He needs to be able to meet with Muslim leaders, face to face, and -- in love -- tell them their religion is the ranting of a madman and without placing their trust in Jesus Christ, they will be seperated from God forever.
If he can't do that, he's got a problem as a leader within Christianity.
I am not Catholic so it is not up to me to apologize (by apologize I mean the original meaning of the word which means 'offering an explaination') for the pope.
Suppose he had not faced toward Mecca: he would have been acting with singularity - that is, been making a spectacle of himself. Muslims may or may not be aware of singularity or of something equivalent in Islam - but, he would by Catholic standards (even if not by theirs) have been drawing attention to himself; which would have been
* very discourteous to his hosts
* entirely out of place in a house of worship (one which is important to them, even if not to him)
* and, completely pointless: he would have likely caused offence, if not to his hosts then to the Turkish public; which would be tolerable (though not desirable) provided that it was possible to achieve some good by not facing Mecca. But what good would not doing so have achieved ?
* Since he is the earthly visible head of a different religion, the presumption has to be that he was not intending to undermine his own religion; and this presumption is supported by his previous acts as Pope: we aren't talking about an honest-to-God five-star abomination like the Assisi meetings.
* No Pope is at liberty to dishonour the Faith by his conduct, of course - any more than any of us is. And facing toward Mecca is of itself a neutral act - it has to be interpreted by the circumstances in which it occurs. For him to face Mecca, does not mean he was dishonouring the Faith unless there are circumstances apart from that bodily action which strongly suggest he intended to dishonour the Faith. Without such circumstances, & without evidence that those other circumstances strongly suggest that he intended that, there is no reason to take that bodily action of turning to Mecca as evidence of anything untoward. For even Popes are entitled not to have their actions judged harshly, if they can plausibly be judged favourably.