Many jurisdictions have laws specifically allowing it, as long as it's disclosed, like in the terms provided by Doyle Auction House:
The auctioneer will not specifically identify bids placed on behalf of the seller. The auctioneer may further bid on behalf of the seller, up to the amount of the reserve, by placing successive or consecutive bids for a lot or by placing bids in response to other bidders.
The auctioneer will not specifically identify bids placed on behalf of the seller. The auctioneer may further bid on behalf of the seller, up to the amount of the reserve, by placing successive or consecutive bids for a lot or by placing bids in response to other bidders.
https://www.doyle.com/terms/