Hm. One poster mentioned "online etiquette", saying that it seems to him that, online, people don't seem to state lines of play when claiming. Well, they quite often don't do it in f2f bridge either. Nonetheless, both online and offline, the laws require a line of play be stated. The wording used is "should state a line of play", so failure to do so should rarely draw a procedural penalty (see the Preface to the laws), but that doesn't mean we shouldn't rule some number of tricks to the non-offending side.
I don't think it very likely that declarer has forgotten that he only drew one round of trump, but is it "at all likely"? Sure. People forget things like that all the time.
There are two trumps out. At claim time declarer has Ax in dummy and T9 in hand. If he plays on hearts, and they break 4-2, he's gonna get ruffed, and he won't be able to overruff. So if the hearts are 4-2, a trump trick to opponents from play on hearts. If the hearts break 4-2 and he tries to pitch his fourth heart on the fifth club, declarer may lose a second ruff as well. For many players it would be careless, but not irrational, to play on clubs at this point, so there's a second trick to defenders.
If you let people claim without a line of play statement, and then give them all the tricks they claimed unless the probability they won't get them all is overwhelming, they won't learn to state a line of play. I would rule two tricks to the defense, IAW Law 70.
Telling them to play on, unless the software
requires they play on (in which case the error is in the software), is clearly director error [Law 68D]. Law 82C now requires an adjusted score, both sides being treated as non-offending. So defenders get their two tricks on their side of the score, and declarer gets them on his side - but it should be made clear the reason for ruling this way is the director error, and if he hadn't said "play on" declarer wouldn't get those tricks.
Directors often do things to make their jobs easier, and so they should - so long as those things are not themselves violations of the laws. As Law 82A says "It is the duty of the Director to rectify errors of procedure and to maintain the progress of the game
in a manner that is not contrary to these Laws." [Emphasis mine].
I've had players (in f2f bridge) ask me to play on, sometimes on very simple claims, because they "can't see how it'll work out". Sorry, but too bad. I'll try to explain it more clearly, but we are
not playing it out.
IMO, the legal basis for
all rulings should be a part of the ruling. It will give the players confidence that you know what you're doing, and will direct the attention of the CTD or appeals committee to the right laws if your ruling is reviewed. At the very least, state which Law(s) you're using.