Why are folks so eager to fire those shots across the bow whenever they get there shorts in a bind.
You would be well advised to understand there are many, many more protections provided the buyer in the purchase of real estate than available to the seller. A buyer typically has more outs than auditorium.
Now, from what I read, the listing agent very likely saw the need to get the buyer to 'fish or get out of the boat!" Such a short acceptance period is unusual and certainly cuts to the real motivation of the buyer. Are they a tire-kicker or a buyer...you've got three hours to decide. The reality is, the buyer already knew the answer, now the seller and listing agent knows also. There are many things a real buyer could do, but from the data you shared, it is unknown if they proceeded in a knowing manner or were so intimidated they exercised the 'out' of no resonse.
The is hard-ball real estate. Nothing you have shared even hints at any shade of impropriety, Your eager use of 'illegal and unethical' suggests you have other motives invested in this story.
Finally, what role are you playing in this? Are you the buyer/seller/concerned community resident or just don't like the listing agent?