I dont think we should change this, FWIW what you are doing in
weights = pm.HalfNormal('weights', shape=10, testval=np.array([0.1] * 10), transform=pm.transforms.sum_to_1)
is incorrect, as HalfNormal does not produce a vector that sum to 1. What transform does is transferring a (vector) of value in \text{R} into something that are in the domain that defined by the distribution.
In fact, the transformation in PyMC3 is defined in this direction only. It is different than package like TFP, where transformation is defined in both direction and you can apply both forward and inverse transformation.