On the basis that a line has to be drawn somewhere, I'm inclined to agree with you. On Friday, there were too many prolonged silences awaiting internet bids, not to mention the aforesaid hammer decision reversal.
At the end of the day, if there is a delay in the internet bid reaching the auctioneer - for whatever reason - that is not his fault. He surely has to call it on the basis of what is in front of him at the fair warning moment. So, as you say, once the hammer goes down, that should be bid successful. It has to be, otherwise the credibility of the sale is compromised and the whole process becomes shambolic.