Why oj-button concatenate its label text to itself when we keep clicking on it,

here is an example of the button in this cookbook button (try when keep click on it) : https://www.oracle.com/webfolder/technetwork/jet/jetCookbook.html?component=pushButtons&demo=events

so on keep clicking the label will be: Button Action 1Button Action 1 and Button Action 1Button Action 1Button Action 1 and so on ?!!