Yes, it's basic tort law. They're not just repacking food items, they're combining them in a non-reversible manner. This essentially makes them the manufacturer. Consequently, they will have opened themselves up to liability for every ingredient they use. (But they may be able to go after a supplier for indemnification depending on the circumstances.

Thanks for the clarification! That makes now that I think about it more. I was thinking more along the lines of product manufacturing, which I think is less stringent (honestly I may be wrong there too).

No. Products liability. Everyone in the "chain" can be liable including retailers and manufactureres.

Supplement companies get sued all the time.