The tent just makes your intent obvious: You planned to sleep there from the beginning. And this is what the authorities want to prevent: Camping in the wild. On the other hand many legislations allow for emergency and high altitude bivouacs, which are inevitable. So the gray area developed where people sleep without a tent in order to escape punishment, as they are just bivouacking. In popular climbing areas in Switzerland they also fine you for bivouacking, as an emergency is clearly not the case (usually people carry stoves and more food/drink than you would "expedition style").

So usually the distinction is planned/unplanned sleeping out in the wild, and often this has been adopted to sleeping with/without a tent, even though the two are certainly not equivalent.

Another reason why bivouac is favored over tent: Bivouacking is associated with trekking lightweight, in which case you do not take stuff to throw away (that would be useless weight). So the potential for littering is generally smaller in bivouacking.

**EDIT** 

Due to the interest this question has spawned and some comments, I informed myself about the laws here in *Switzerland* regarding this issue.  
Somewhat surprisingly on a national level Switzerland knows a "Jedermannsrecht": Article 699 of the ZGB (civil law) states that access to forest and meadow within the locally common extent is allowed to everyone. Fishing and hunting is excluded from this and regulated on the level of "Kantone". This is even more explicit in article 14 of WaG (laws concerning forest): "Kantone" have to make sure that the public can access forests. Again restricted locally to preserve nature and events need approval from authorities. Of course this is restricted by further laws about conservation areas, but it should be common place that in any kind of conservation area (there are lots of different kinds around here...), you have to inform yourself whether its allowed or not and in doubt, you don't.  
So one might think "Why all this fuss about whether you can camp or not in the wild, this says you can...". The problem lies in the details, which in this case is the access up to "locally common extent". As Switzerland is based on federality, meaning very strong independence of the "Kantone" and of municipalities, they can basically all choose what they consider "common extent". So basically in Switzerland you either ask in every municipality whether you are allowed to or not or you adhere to commonplace rules and the so called eleventh law: Don't get caught.  
Still I was not aware about the extent of legal backing for free access to woods around here: Basically you cannot fence any forest area even if you own it privately, as access to the public is granted (and walking through it is the basic kind of access, so you can't exclude it as "locally uncommon"). I don't know whether this will help anyone, I justed felt like writing it down.