Since watchOS 2 was released by Apple (September 2015), developers have been able to offer apps that access the built-in heart rate sensor on Apple Watch.
The issue with the apps you mention is therefore not a question of Apple needing to allow anything (it already has). Rather, it is a question of the app developers needing to build that feature into their apps.
MapMyRun
The developers of the various MapMy apps state that support for accessing the watch's heart rate sensor is currently "in development". If you would like to let them know that you are keen to see this feature added as soon as possible, you can do so here:
https://support.mapmyfitness.com/hc/en-us/requests/new
Nike+ Running
A workaround for the Nike+ Running app is to record activities using both your Apple Watch (start recording there first) and via the Nike+ Running app on your iPhone.
Tracking your workout via the watch's built-in Workout app will cause your heart rate to be monitored continuously for the duration of your workout. If you allow the Nike+ Running app to read Heart Rate data from the Health app on iPhone, it will be able to access - indirectly - heart rate data for your workout:
- On your iPhone, in the Health app, tap on Sources (tab) > Running > turn on Allow "Running" to read Heart Rate data.
You may wish to let Nike know that you would like them to add support for the watch's heart rate sensor:
https://secure-nikeplus.nike.com/plus/support/#ask/