People are walking the marathon for a cause.
People are walking the marathon for a cancer charity.
Just because people walk for a cause doesn't imply the cause is for cancer charity.
exists e1 x1 x2 x3.(n_marathon(x2) & for(e1,x3) & n_cause(x3) & Theme(e1,x2) & Actor(e1,x1) & v_walk(e1) & n_people(x1))
exists e1 x1 x2 x3 x4.(n_marathon(x2) & for(e1,x3) & n_charity(x3) & of(x3,x4) & n_cancer(x4) & Theme(e1,x2) & Actor(e1,x1) & v_walk(e1) & n_people(x1))