They’re just counterfeit proxies. It is not like custom cards. The Mystery Booster cards actually exist and there is nothing to gain from not using the official cards that support Wizards in making future Magic sets.
I can't speak for others, but I think the original test cards look like shit. I like them for what they are, a fun, little experiment, but I skipped acquiring any because of their aesthetics. I would consider putting into my cube this version of Frogkin Kidnapper, though, since it looks so much better with the rest of my cube. I'm not costing WotC money, since the alternative is me not buying any of those cards anyway.
I know there are people who proxy the expensive cards in their cube. You could argue that they're costing WotC money too, but the alternative often is that they don't own a cube at all, which means they wouldn't acquire the cheaper cards either, and maybe would play less Magic as a result, thereby actually lowering WotC's revenue. Also, it's not like we owe WotC our money. Rather than chastise people for using proxies, I think it's good to look at people's motivations for proxying, because it's not always as black and white.
The only instance in which proxying is objectively wrong is in a tournament setting. If you want to compete in a WotC-sanctioned event, the rules are clear, you have to buy the cards. WotC has every right to set this requirement as well. Though, even there, I've seen plenty of Vintage tournaments that allowed up to five proxies, because having players is better than not having any, and getting all the power is prohibitively expensive.