Agreed.... but in real life, you are already starting from a warm tank so even a small heater can raise the water temperature to lethal levels.
Real life is real different, depending upon where the life is being lived. If a warm tank is in a cold room, it is losing heat at a rapid rate; the greater the temperature difference, the more rapidly that heat is lost. A small heater in a warm tank struggles to merely maintain that temp, if it can do so at all. Saying that you are starting with a warm tank and therefore the heater will continue raising the temp from there ignores the fact that heat is constantly pouring out of the tank into the surrounding cold air and must be replaced just to maintain the temperature. Increasing the temp from that point requires a great deal more energy (i.e. increased heater wattage) than merely achieving that temperature.
A given wattage heater can only raise the temperature of a given tank volume by a limited amount above the ambient air. Once that temperature is achieved the limit has been reached; the only way to raise it further is to increase the total wattage of heaters being applied, to raise the ambient temperature of the room, to insulate the tank and slow down the loss of heat, or some combination of these.
If a heater is able to raise the temperature of a tank to lethal levels, it is simply too large for that tank in those conditions. As long as it is properly controlled, it's fine; but ideally a heater should be of a wattage just sufficient to achieve the temperature you want, without going to dangerous levels if uncontrolled. Easier said than done, but nothing is ideal...
I can heat my 360gallon tank with a 500watt heater during the winter; it can, if run continuously, only get that tank up to about 68-70F if my basement goes no lower than about 58F. But...if the basement temperature were to drop to 50F, which is certainly possible, then the tank temperature will be about 8 - 10F colder as well. That tank will never get dangerously hot with that heater; even if stuck "on" it simply can't add enough calories to counteract the heat loss to the air and also raise the temperature dangerously higher. It's a fine line; simply removing the clear poly cover from the tank for a few hours causes a drop of several degrees due to increased heat loss.
If even small heaters could overheat large tanks...well, we'd all be using them, since controlling them properly would make them safer to use. You need a heater large enough to raise the temp of your specific tank the number of degrees that you want it be higher than your ambient air. Most people go way overboard and use heaters larger than necessary, thinking that they are somehow "better". They're not. Too big is just too big, but can be made to work if controlled properly. Too small is just...too small.
