Actually, you should be referring to Singapore laws when trying to define litter and theft, and not "answers.com".
1.  Littering:
Quoted from ENVIRONMENTAL PUBLIC HEALTH ACT:
Prohibition against throwing refuse, etc., in any public  place
17. (1)  No person shall 
(a)  deposit, drop, place or throw any dust, dirt, paper, ash, carcase, refuse,  box, barrel, bale or any other article or thing in any public place;
Hence, under the law, dropping or placing a tissue packet on a seat in a hawker centre would fall under Section 17(1)(a) above.
Now onto theft.
2.  Theft
Quoted from PENAL CODE:
 Theft.
  
378. Whoever, intending to take dishonestly any movable property out of the possession of any person without that persons consent, moves that property in order to such taking, is said to commit theft.
    Illustrations
  
  (g) A finds a ring lying on the high road, not in the possession of any person. A by taking it commits no theft, though he may commit criminal misappropriation of property.
  
Hence, there is no theft by taking a tissue paper packet lying in the middle of the food court.
Now onto criminal misappropriation
 3.  Dishonest misappropriation of property.
  
Quoted from PENAL CODE:
403. Whoever dishonestly misappropriates or converts to his own use movable property, shall be punished with imprisonment for a term which may extend to 2 years, or with fine, or with both.
    Explanation 2.
  A person who finds property not in the possession of any other person, and takes such property for the purpose of protecting it for, or of restoring it to the owner, does not take or misappropriate it dishonestly, and is not guilty of an offence; but he is guilty of the offence above defined, if he appropriates it to his own use, when he knows or has the means of discovering the owner, or before he has used reasonable means to discover and give notice to the owner, and has kept the property a reasonable time to enable the owner to claim it. 
    Illustrations
  (a) A finds a dollar on the high road, not knowing to whom the dollar belongs. A picks up the dollar. Here A has not committed the offence defined in this section.
  
Hence, in order to avoid criminal misappropriation, all you need to do is to take the tissue packet, sit on the seat and eat your food.  When the person comes back, return the tissue packet to him.  Viola, criminal offence avoided.
Conclusion
So, there you have it, the person leaving the tissue packet is guilty of an offence under the Environmental Public Health Act, but the person taking it is not guilty of any offence of theft, nor criminal misappropriation, provided the above recommended steps are taken.
Finally, since both parties are pots calling the kettle black, then both are equal.  Hence rebutting your earlier attempts to try to say that the tissue packet reserver is not in the wrong, and the tissue packet taker is in the wrong.