It's not even about getting a discount. That implies we're trying to pay less for a product we want. It's about being required to pay extra for a product we don't.
If I buy the pdf then the Foundry module is $21.
If I don't want or need the pdf and just want the Foundry module, then I have to pay the $39 extra for the pdf anyway.
It's framed like a discount, but when I have to buy the pdf either way then it's not a discount. It's just being forced to buy a product I don't want for the privilege of buying one that I do.
The only reasonable explanation for setting these prices is to squeeze as much money as possible out of existing, paying customers. Otherwise just set the Foundry module at $21 and the pdf at $39.