You have fairly well described my concerns with how undertank heat sources are typically utilized by reptile keepers. And kudos for you for thinking abou this enough to check the actual air temperature above the heat source and not only at the heat source.
Whether this is appropriate for a terrestrial snake largely depends on the species in question. In general, however, I'd say no -- any species that would "need" and utilize a 90* surface temperature in a cage would likely befefit from air temperatures warmer than 74*.
All to often keepers use a single undertank heat source to warm a simple cage with minimal furnishings. In a relatively cool room one of two extremes is often found. Most common is the keeper only pays attention to the temperature at the heat source and the rest of the cage ends up being cool. Even worse, the shift from the high temperature to the low temperature tends to be very abrupt and not occur gradually over the length of the cage. Not surprisingly, snakes in these conditions spend a lot of time basking at the heat spot.
The other extreme is that the keeper pays attention to the air temperature with a probe mounted a few inches off the ground. A thermostat regulates that spot and that spot only and the ground area ends up having to be heated to super hot temperatures in order to satisfy the thermostat. Not surprisingly in these situations snakes rarely are found laying directly on the heated area as it's too hot. But in the case of some snakes, like burmese pythons, severe thermal burns can occur if the snake lays on the heated area.
The solutions to this can be quite simple. First, if you have a screen top on the entire cage consider sealing off part of it with some aluminum foil or perhaps even build a more solid lid for the tank.
Next, increase the thermal mass in the cage, especially near the heat source. Flower pot hide boxes, rocks, heavy ceramic water bowls, or even tiles can be placed in cages to increase thermal mass. One word of caution is to not place a water bowl over the heat source unless the species in question requires high humidity.
Doing the above will help distribute heat throughout the cage much better.
Another great solution for tanks with screen lids is to use a dome reflector fixture with a regular incandescent light bulb. A step above this is the use of a ceramic heat emitter. In both cases make sure the fixture is rated for the wattage of the bulb or the heat output of the heat emitter.
For even more money there are also radiant heat panels.
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Current snakes:
0.0.1 Gonyosoma oxycephala - Java locale (green)
1.2 Gonyosoma oxycephala - Jave local (green)
2.2 Gonyosoma janseni - Seleyar locale (all black)
1.2 Gonyosoma janseni - Celebes locale (Black & Tan)
