The "standard" you're referring to is simply not as good. Older screw heads are dated designs that are more likely to strip out than a torx or hex design. They're mostly still in use because the benefits of being common outweigh the downsides for many applications, especially in materials like wood that are more forgiving.
Torx is an accepted standard (just not one commonly found in homes.)
Lots of manufacturers use torx screws in locations where there is very little chance a home user will ever see them. They do it because it makes for a better product initially and easier repair by a professional later on.
You're right. I was thrown off by the message higher in this thread that called them Torx screws. These are not Torx, they're pentalobes, and yeah, it's a non-standard by any reasonable measure.
That said, I'm still not agreeing with all this carping. You buy a product that's not advertised as having user-serviceable parts inside, then complain it's hard to service. I don't see folks complaining that the inside of their Android phone or Dyson vacuum are hard to hack, and that's what the iMac is equivalent to - it's an appliance. You trade away the flexibility of a desktop form factor for the coolness of the all-in-one.
This is how I imagine the story playing out: Someone at Apple thought it would be a good idea to have fewer of those cases where someone cracked open his laptop for fun and ruined it and then was bitterly disappointed when Apple wouldn't fix it on warranty. That's a lot less likely to happen when nobody has the correct screwdriver just in the basement and has to order it from somewhere.
Whether that's the right or a useful thing to do I don't know but it's also no big deal.
Torx is an accepted standard (just not one commonly found in homes.)
Lots of manufacturers use torx screws in locations where there is very little chance a home user will ever see them. They do it because it makes for a better product initially and easier repair by a professional later on.