If Apple buys back shares for one billion dollars, then the value of the company is reduced by one billion dollars. That's compensated for by the fact that there are now fewer shares, so the value of the company is divided by fewer shares.
If Apple buys shares at exactly what they are worth, that's pointless. If Apple buys shares at a higher price than they are worth (because the share price is too high), that's bad because it reduces the value for the remaining share holders, and I don't think you can make an argument that Apple owes anything to share holders selling their shares. If Apple buys shares at a lower price than they are worth, that's a good thing because the value for the remaining share holders goes up.
Apple is quite high at the moment, so buying shares is not that good an idea. If Apple dropped to $90 or $80, then they should buy shares.