This is a general problem, not limited to the Behringer Studio 50 USB monitors or the Macbook Pro. This problem can be explained by the fact that studio equipment is aimed at two different types of users and use cases, and these monitors seem to fall between two chairs.
In a low-budget home studio, you will usually find all-in-one monitors with a digital input (USB or S/PDIF), built-in digital-to-analog converter, built-in amplification, and the necessary controls for volume and often also balance and tone. You simply connect these to a computer and you have a complete monitoring solution.
An example of this type of monitor is the Behringer MS40, which has a power switch, headphone output, bass and treble knobs, and volume for both analog and digital input on the front panel.
In a professional or high-budget amateur setup, you will usually find separate specialized devices: a high-end digital-to-analog converter, a "monitor controller" with volume controls and selectors for different speakers, and active monitors or an amplifier and passive monitors.
An example of the type of active monitor used in such a setup is the Yamaha HS8, which has no controls on the front panel, and a level knob on the back which is used to correct level differences between monitors, but doesn't really offer full-range volume control.
As you will notice, the Behringer Studio 50 USB combines elements of a high-end studio monitor with those of a budget monitor. There are no volume controls on the front panel, but there is a digital USB input. I can only assume that the USB connection was added as an after-thought, to a speaker that was originally supposed to be used with a separate digital-to-analog converter and volume control. As it is, the USB connector isn't really useful because, as you have discovered, there's no good way to control the volume of the USB audio, and lowering the volume of the digital signal before D/A conversion reduces the bit-depth and thus the audio quality.
Recently, manufacturers of audio interfaces have realized that many customers have a mixed setup that doesn't fall neatly into either the budget or the professional use-case, and there are now several affordable interfaces that are clearly designed for people who want something akin to the Mackie Big Knob to control the monitor volume, such as the Focusrite Scarlett 2i2:
The easiest solution for someone who has monitors with no built-in volume control and a limited budget would be to buy a simple external USB audio interface like the Focusrite or the Steinberg UR22. This will also give you better D/A converters than those built into affordable Behringer monitors, and additional recording functionality.
