Earlier quoted context omitted.
It's not that complicated. They don't pay the tariffs. The person receiving the package does. Many carriers will slap you with the tariff charge, a brokerage fee, and then send you to collections if you don't pay it. The vendors don't care because they're making the sale and the tariffs are the other person's responsibility. Caveat emptor.
The person you are replying to isn't claiming that the seller pays the tariffs, they are saying that it's not in the seller's interest to notify buyers of the tariff charge because it's essentially free anti-tariff messaging once buyers are hit with the sudden fees.
They're not deliberately plotting some anti-tariff surprise campaign. They're just doing business as usual.