How can you pass a List<objects that implement an interface> to a method?

For generic wildcards the keyword extends works for both classes and interfaces:

private String getListAsJson(List<? extends JsonEnabled> list) { ... }

extends has slightly different meaning when used for defining generic bounds - it essentially translates to "is, or extends, or implements".

Why don't just use

private String getListAsJson(List<JsonEnabled> list) { ... }

?