Two reasons: the first is that I have no freaking idea. I'll give this some more thought and get back to you. The second is that I'm not saying it would be impossible to reject God if he appeared to you, I'm saying that it would be a strong violation of your free will to believe or disbelieve. In the Old Testament, salvation was largely a function of following God's laws to the letter (sacrifices, etc.) and generally doing your best to be righteous, but even then he didn't provide irrefutable proof of his status as the One God.
Can you go into more detail about why it providing evidence for something 'violates' our free will?
For one thing, it seems to me that belief isn't something you actually choose anyway. Either you are convinced that something is true, or you aren't--there's no choice in the matter. You could ACT as though you are convinced when really you aren't, but what you actually believe is not something you get to control.
Could you, right now, sincerely and truly believe that Santa exists? Is that a choice you can freely make? Can you say, 'From this day forth, I will believe in the jolly old elf with all of my heart' despite all of the evidence and knowledge you have accumulated that suggests it simply couldn't be so?
Could you, as a christian, sincerely decide to stop believing in god right this second?
If not, then I would posit that belief has nothing to do with free will.
What does violate free will is withholding information from someone who is making a crucial decision. For example, suppose a man and his wife are getting intimate. The man (or woman, it doesn't matter) has been unfaithful, and has a sexually transmitted disease that is fairly dangerous, but does not tell his partner about it. If the other person was in full possession of the facts, she would probably not choose to have sex with him. By not providing her with the truth, he denies her ability to make a free and informed choice and is also putting her at risk for illness or even death.
Presumably an all powerful being possesses the power (since it is all powerful) to personally provide every single person with proof of its existence that will convince them of the truth, regardless of how skeptical or disbelieving that person was previously. With this greater knowledge in hand, everyone could then make informed decisions about whether or not to obey/love/worship this being. By not providing us with the truth, such a being (if one existed) is denying our ability to make free and informed choices, and is also putting us at risk for eternal torture.*
In the modern era, salvation is moreso a function of belief in Jesus. Of course, you still have to accept him (it's not sufficient to believe but be a blasphemer), but the heavy lifting is done. Also, I don't really know of anyone who could consciously and rationally reject a being that they knew would damn them eternally for doing so.
If all God wanted was automatons that loved Him, he could make a bunch of robots.
Well, if god did exist as you described, I certainly wouldn't be a loving robot. I wouldn't reject him (no, I'm not ready to be tortured forever to prove a point to a psychotic) but I would only obey out of fear, not love. Rather like a child in an abusive relationship, constantly under threat and willing to do anything to avoid torment.
*Unless you're a universalist.