Why not? What's the downside?
For starters you don't know what it is adhered to the board with. If that canvas every bubbles, warps, needs repair, etc., or the more likely scenario, if the board itself warps, takes on moisture, is damaged, needs repair, etc. good luck removing the canvas from that board. If the canvas is stretched, it is easily tightened in less than a minute if it gets a wrinkle or sag, and can always easily be removed and simply restretched for any reason. I've had customers call me almost in tears with broken stretchers from a move and they thought their canvas was ruined. Nope. Just an hour or so to restretch and it's good as new. Go to any fine art museum and you won't find many, if any, canvasses adhered to a board or glued to anything.
Again, I suppose this cheap method is fine for their target audience, and it's the method they probably have to use to keep it cheap. I would definitely glue some canvas or prints to board for any sort of 1-2 day promotional event where your stretched canvas could get damaged lugging it around. Maybe even order some of these cheapos for that purpose. They are cheap and pretty much disposable.
The 20x24 portrait canvas my clients pay upwards of $500 for, I'm simply not going to deliver to them a cheap canvas adhered to a board. Not good for my client or me, or the long term viability of my business. YMMV.