I pondered that when I was shopping for a torx set, but I figured they were probably a little weaker than the regular torx keys - as a matter of fact there was a set on sale that had a broken key already (it had lost one of the splines on one of the smaller keys).
From the look of the anti-tamper Torx screw I have here (which I still haven’t undone yet), I’d say that the cylinder of metal in the centre of the pattern probably supports the driver as it turns the screw - making the driver quite durable in that application, whereas if you use the anti-tamper driver in a regular head, there’s nothing to stop it collapsing inwards under stress.
That’s what I thought, especially as they were Halfords cheapos. So sadly the set of tamper-proof torx keys will have to wait for a genuine occasion. 
Yes, and they are making an even newer type of torx wrenches that are not backwards compatible. They kind of work, but are a little sloppy, so keep an eye out for them.
Here’s the quote from Wiki;
And don’t buy cheap tools from Harbor Freight and the like. You get what you pay for. Those tools are usually not as durable, not as hard, or not the exact fit. You don’t want to strip the head off a 3.5mm screw - thems are a bitch to get out once the head is mangled.
You have obviously not met one of these.