Why are particular laser-related skin products not recommended for darker skin tones?
The reason for laser hair removal/growth products not being safe for darker skin tones is the ability for darker skin tones to absorb more light from the laser than lighter skin does BEFORE the light is able to reach the depth at which the hair follicle is located. Light from lasers must pass through the epidermis where melanocytes are located before they reach the hair follicle. 



More melanin means increased ability to absorb wavelengths as seen in the curve below. Laser products such as Theradome (678nm) and Silk'N (475-1200 nm) have shorter wavelengths, which are highly absorbed by melanin, and therefore can damage skin in darker skin tone patients. Safer products like Nd:YAG are those that have higher wavelengths at 1064nm and can penetrate the skin without over-absorption by melanin. 



Disclosures: I own no stock in any of these companies.

References:


http://www.theradome.com/press-release/
http://www.amazon.com/Silkn-Flash-Removal-Device-Cartridge/dp/B0093BOR4W
http://www.nature.com/jid/journal/v122/n2/full/5602198a.html