For HTML5 apps that use the REST API the user-ID must be known. Also, without using the REST API it is useful to allow HTML5 apps access to user properties such as the users firstname or lastname to create a personal greeting page for example.
The user properties exposed in the HTML page can be controlled usin the user property context with the identifier org.olat.core.commons.fullWebApp.BaseFullWebappController. This can be configured in Administration / Customizing / User Properties / Contexts in the admin GUI. To disable the feature, all user properties can be unchecked here. By default the firstName and lastName are enabled.