Premiere will not automatically change the speed without user interpretation. If you edit 50 fps footage in a 25fps timeline Premiere will only play every other frame. If you would edit 25 fps footage in a 50 fps timeline it would show every frame twice. 100 fps footage would show every fourth frame in a 25 fps sequence.
When you interpret footage you are setting the clip to run at a different speed so the duration and audio will change.
When you shoot at any framerate, the camera adds metadata for that framerate. That’s how the NLE knows what the “correct” framerate is. Since you shot at 50fps, the camera has set the video metadata to 50fps. Premiere sees that data and adjusts the clip playback in the timeline so that it runs properly against real time, as Tero has described.
If you want something to run at half-speed, you can either change the clip speed in the timeline (Speed/Duration or Time Remapping), or you can tell Premiere to change the assumed clip framerate using “Modify>Interpret Footage.” Interpret changes the clip itself and must be changed prior to putting it in the timeline.