Piracy

Unfortunately that's spotty evidence at best. Game developers generally don't sell their own keys, they sell them through distribution platforms like Steam. Gamers and resellers both pick up keys from distribution platforms. A percentage of these are legit, a percentage are fraudulent, however the game dev has no way to actually tell:

1. How many legit sales are to resellers
2. How many fraudulent sales are to resellers

Distribution platforms don't make you check a box that says "I'm a reseller", so there's no way to know if the net income from legitimate sales to resellers is less than the cost of fraudulent sales to resellers. The only thing game devs have to rely on is signal from chargebacks, and a chargeback makes a lot more noise than a legitimate sale.

I think it's very possible that the few devs that tell you to pirate instead of using resellers are misinformed, and of course they are, because they don't own their own distribution line. Fraudulent sales cause more noise than legitimate sales which causes people to jump on the "pirate over resellers".

But regardless of which side of this argument is correct, telling people to boycott an entire sub-industry based on claims with speculative evidence is ridiculous.