I can't quite understand why they would do this, why add it.
Very odd.
They're not just randomly adding tallow for kicks, as part of the industrial process for these plastics you NEED a tiny amount of either tallow or plant based alternative.
There is far more tallow produced by the meat industry than needed by industry. So they can either use the cheap and readily available waste product tallow, or that tallow will be wasted and destroyed and a plant based alternative can be made, which doesn't affect the number of animals killed (they were all dead for the meat anyway), but does have an increased environmental cost.
I know which seems the sensible option to me.