I know specifically of a couple of middle eastern airlines that have very clear and specific rules regarding carriage of UMs - it's simply a case of RTFM. Not seating them next to male pax is NOT insinuating that all males are sexual predators for christs sake, just that 99% of all sexual predators are male. Unless you have time to vet all your pax thoroughly with background checks, references and notes from their mum, it's just easier to take one potential out of the equation.
Regarding pax who refuse to eat on board - definite sign of drug smuggliness, and they should be prevented from going to the bathroom as well in case they try and jettison their drugs before deplaning. In fact, check out that fat bastard in 36G, I'm almost certain that roll of fat he's carrying on his waistline is actually a couple of keys of Colombian snowflake strapped to his body.
I'd also watch that kid in 17C with the lollipop, I heard that's how LSD was transported in the 80's.
Don't forget the grungy couple in the homeknit jerseys on 25AB, that unwashed odour is definitely a masking agent for the stench of reefer that I'm sure they have stashed in their dreads.
Logical reasons why people might not eat airline food, not including the Jenny Craig drug mule diet:
1: Airline food tastes like cardboard warmed over.
2: They spent the last few days partying it up, and hangovers don't mix with omlettes that are turning green.
3: They didn't get the special meal they demanded and are on a hunger strike in protest.
4: They are concerned about their allergy to nuts or similar and don't want to risk eating something that might cause their airway to close up.
5: It's ramadan/lent and they are fasting.
6: They are vaguely health conscious and don't feel like eating 2000 calories of fat soaked carbs.
7: Returning from their dream holiday in Montezumastan, they picked up the worst case of food poisoning since Yo Sushi offered chicken sashimi, and are incapable of eating anything.
8: Their bodyclock is telling them its 0300 and they just don't WANT to eat anything at that time.
As crew, a pax not eating can be symptomatic of a lot of things, smuggling drugs probably shouldn't be your first assumption.