Altaïr was known to stick his neck out for strangers in danger, people he thought might be innocent but preyed on by corrupt city guards, tyrants, etc. Such a simple request surrounded by mystery, kidnapping, magic...?

"If they wished for something so simple, they could have asked without all this suspiciousness. And taking my things. Many would say they wish to protect life. I suppose what I want is to know the truth hidden from me." A simple enough desire, and not specific enough to betray any future plans he might make. He didn't know this stranger, after all.