Children inherit opacity. It’d be weird and inconvenient if they didn’t.
You can use a translucent PNG file for your background image, or use an RGBa (a for alpha) color for your background color.
Example, 50% faded black background:
<div style="background-color:rgba(0, 0, 0, 0.5);">
<div>
Text added.
</div>
</div>