Spicy soy, seed bitterness and placental capsaicin
It's not that they contain no capsaicin - they do contain some - about 5 mg/g dry weight, compared to 7 mg/g for the main pericarp (the fleshy part that you eat), but most of it is in the placenta/veins (64 mg/g). 

The spiciness is attributable to the presence of capsaicin and other polyphenolics in the seeds, and probably some placental contamination too. I have no reference for this, but I doubt that many industrial methods of extracting the seeds will remove all of the placenta from the seed or work so carefully that some of the juice from the placenta isn't coated onto the seeds in some manner. 

It is also likely that you are adding a relatively high amount of seeds to a small volume and infusing from there. In addition you are breaking the seeds up a little so you will be able to extract more of the heat than you might with intact seeds.

Normally when I make the chili/soy sauce, I add some sesame oil to help extract the capsaicin and act as a vehicle to enhance the flavour/heat - perfect with some Jiaozi/potstickers.