What are these normal "Under Offer" == "Offer to you to Gazump" rules that you are referring to? Sold subject to contract means nothing in England and Wales except that the seller has agreed to sell it to a buyer at a certain price. At this point, neither party is locked in to a contract and either can drop out. Unless the buyer has stipulated as part of the verbal agreement that the property be taken off the market, the EA is perfectly entitled to continue marketing it. SSTC and Under Offer are effectively synonyms. If someone comes along with a better offer, the vendor is free to accept it.
With so many deals failing these days because the buyers cannot raise the mortgage, I fail to see why the vendor should be narked at the EA for continuing to advertise the property for sale. Indeed with house prices falling, the buyer may have accepted an offer which is considerably less than what they were asking and the EA may indeed be fishing for someone prepared to pay more.
One possible reason for marking it as 'Under Offer' on Rightmove is that marking a property as SSTC will remove it from many people's searches. If you drive by on the other hand, you will see the SSTC sign.
I have to wonder why, if you were really interested in buying the property, you didn't put in an offer at the price which was right for you? Did you expect them to advertise it at that price before you would consider? For all you know, they may have accepted a price which is less than you were prepared to pay.