It would be very rude to reject any offering on a doorstep. You just say thank you and wish them a Happy Halloween, and that goes for wrapped and shop bought things as well as homemade. When you get home you can unload/swop/give to grandad whatever you don't like and nobody gets hurt in any sense of the word.
'This is about people taking their children to knock on participating doors, asking for things, being given things, then refusing to eat them. What is the point in knocking and asking in the first place, if you are just going to throw the things in the bin (because they might not meet your 'exacting' requirements)??? Bloody rude, IMO'
You have missed the point of Halloween here if you think it's about cleaning your plate, so to speak, or feeling obliged to eat every last piece of candy.
No child needs a single piece of chocolate or packet of sweets, let alone a pillow case full. They are not going around asking out of need. No child needs to consume the amount of candy he or she will receive at Halloween, either that night or in the weeks afterwards if she paces herself. Back when I was a child in Dublin, no child would have needed the huge bag of apples and nuts I used to drag home either.
It's not begging. And there are no 'exacting requirements' either. Plenty of children don't like many flavours of sweets, or chocolate with/without nuts, and some will not be allowed to chew gum or anything they could choke on like gobstoppers -- what skin is it off your nose if you give those items, as long as the children are having fun in their costumes doing something completely different from normal everyday activities, are polite, say thank you, and all parties concerned move on with their lives? Do you know the ultimate fate of anything you give out at Halloween, be it shop bought or homemade?
What the children are asking you to do is participate in the celebration of All Souls Day/Samhain with the gesture of giving a piece of candy. The 'trick or treat' bit refers to an alleged American custom of vandalism (a trick) if a treat isn't given, but actually in Britain and Ireland the tradition is much closer to the Irish and Scottish guising, with no threat implied. Traditionally, the donations of apples, nuts or candy would go towards a community party around a bonfire or firework display. Fireworks were prohibited in Ireland when I was a child so it was always a bonfire.