When designing shelves of this nature, the manufacturers understand that these shelves are hanging with no external support. Therefore, they are specifically designed to be reinforced. This is done by adding attaching screws into the wall and the internal brackets are made from durable materials. They are also extended enough to give excellent support. With this type of design, you are able to put heavy items on these shelves without worry.
Floating shelves look fragile to the naked eye because of the way they sit on the wall. All the workings of the brackets are hidden so it seems as though they are floating there on the wall. In reality they are usually much stronger than other types of shelves which are supported with traditional brackets and screws.