Allergies are an inconvenience to others but also they're not exactly the highlight of the sufferers life either, so whilst I find a.few things become a challenge with some, they're not doing it on purpose.
It's strange, I don't really recall being told I had to clear my plate, I do recall being told not to waste food - as in don't take more than you can eat, and to eat my vegetables because they're good for me... those 2 things have stuck with me, and as a child I knew what I liked and what I was less fond of... (sorry veggies and vegans) - but I love meat and of course gravy and spuds and those sorts of things too, so if I had (and still do) a roast dinner, I'll eat all the veggies first "because they're good for me", then I'll eat the bits I enjoy next, always clearing my plate...
Even now if my wife makes me a sandwich or something with salad on the side, I'll wolf down the salad to get it out the way before I move onto the enjoyable (for me) bit.