I'm not an Illustrator expert, but even I know enough about the program to tell you this. It is entirely possible to put multiple objects on the same layer.
It is expected. If you don't explicitly add more layers to your document, all the objects you create go into Layer 1.
Prey, I will open up Illustrator right now and put multiple groups on a single layer, if you want me to. I'm betting several other members could do it as well.
Done so many times, including this morning.
Adobe Illustrator's data model presumes only a single page. That's why Adobe introduced InDesign for laying out documents having multiple pages, but using toolsets similar to Illustrator. A page in Illustrator is a list of one or more layers. Each layer contains zero or more objects. There is a special aggregator object, the Group, that may contain displayable objects and other groups. Hence the document structure in Illustrator (and InDesign) is infinitely hierarchical, and the Layers dialogue represents this deep hierarchy as a tree control. The top level of the tree are layers. All the rest are either objects or object groups. Each entry in the Layers dialogue is a member of the hierarchy, not a top-level hierarchical parent. The color-coded bands between the object visibility control and the object type/name field is an indicator of the layer to which the object belongs. Birthers seem to be mistaking that indicator as a designation of the object's rank in the document hierarchy.
Adobe Photoshop's data model is that of an image with global color space and image depth properties, made up of a list of one or more layers. Beginning with Photoshop CS4, a layer group could be added so that layer blending properties could be applied to more than one layer simultaneously. While the Photoshop document model is now thus infinitely hierarchical, in practice Photoshop documents exhibit shallower document hierarchies. This is because the only displayable data element in Photoshop's data model is the layer, as opposed to Illustrator which offers many displayable object types.
An entry in the Photoshop Layers dialogue is either a Layer or a Layer Group. There is no common entry subordinate to the Layer. Layers may be either explicit pixel maps or Adjustment Layers, which apply a parameterized algorithmic transformation to the cumulative display image according to the layer blending mode and image mask.
Hence Mara Zebest, familiar only with Adobe Photoshop where each entry in the Layer dialogue is most likely a Layer, commited a common layman's mistake while interpreting the Layers dialogue in Illustrator, a program with which she is not familiar. Rather than understanding that she was seeing a document hierarchy of a depth typical for Illustrator but atypical for Photoshop, she naively interpreted each entry in the Illustrator Layers dialogue as a Layer, much as an experienced Photoshop user might. This is, however, an incorrect and inexpert interpretation of the Illustrator data structure, as a number of subsequent subject-matter experts have pointed out.
Adobe PDF's data model is similar to InDesign's in that it composes a Document as a Catalog of displayable Pages and non-displayable support objects like fonts and filters. But it is dissimilar in that a PDF Page is a list of zero or more Objects or XObjects; there is no intervening Layer aggregate.
It is inappropriate to say that a PDF has, by default, one layer analogous to the default Layer 1 in Illustrator; PDF simply has no concept of layering as we define it in Illustrator.
In later versions of the PDF language standard, a first-class mechanism was provided to
optionally group objects for various purposes such as for interactive display or to remember the content-grouping of content-authoring tools. But as previously discussed, this mechanism is not equivalent to first-class layering and grouping in Illustrator, Photoshop, or InDesign. Illustrator does not import content so grouped as different groups or layers -- Illustrator always imports PDFs into one layer. It is in any case irrelevant to Obama's PDF, which is version 1.3 and supports no inherent grouping whatsoever.