[Avodah] zilzul shabbat

[Sholom Simon via Avodah]

I'm having a hard time wrapping my head around the suggestion that a
kosher lamp is zilzul shabbas. 

Isn't just a nice (and -- significantly
-- *safer*) way of covering a lamp? 

If one were to make the argument
that its convenience might make it zilzul shabbas, might one respond
with "but safety"? 

Why do the rabbis deem the following to not be a
case of zilzul yom tov: cooking a large pot of food on the afternoon of
yom tov rishon, "in case people might stop by", and "I'll eat some of
it" knowing there will be leftovers? 

-- Sholom 
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