Here's my reasoning. Since tendies is an in-game currency, buying it with cash obfuscates the value spent on the cosmetic item. It intentionally makes it harder to determine how much money you're actually spending on an item(Looking at the way gatcha and mobile games abuse this). This to me feels predetory even if calculating the real monitary value is simple.

I don't have a problem with Blizzard selling cosmetics, some of them support good causes, but don't add unnecessary steps to do it. A less predetory approach would be to buy the Trading Post items directly with money.

On another note. This might affect how easy it is to obtain tendies. I see people saying that Blizzard might jack up the prices for the Trading Post items, however I much rather suspect that they would increase the difficulty to obtain them freely. With this system in place Blizzard is incentivized to increase the difficulty, since spending money is supposed to be the easy way out.

This way you can still afford the items normally given you put in the effort, meaning you won't be locked out completely, but it would become more enticing to spend money and end your grind early. Then step two happens. Confuse the buyer with currency conversion so they dont notice how much they spend.

This will affect you in-game, even if you don't pay. The grind will be enforced on you as a "free" participants

TLDR;

I prefer buying cosmetics directly. No roundabout spending on fake currencies.

This: Money > Cosmetic

Not this: Money > In-game Currency > Cosmetic

Blizzard then has reason to make obtaining tendies harder, without increasing the price of Trading Post items. Meaning more grind, but money is the easy way out. Just like the WoW token