While I don't disagree that the air/ground sensing system may not be perfect, let's not forget that the first-level "killer item" is to set the flaps, as described in the before-take-off checklist. TOWS is the second-level safety feature for that one. Then we may argue if an air/ground sensor failure alarm that would be a safety feature for inoperative TOWS is on the same or the next level. Anyway there is a limit for how many levels of fault detection, redundancy and fault tolerance are practical before the solution becomes more failure-prone than the item it's designed to protect.
Some posts mention that the MD80 TOWS shall be checked before every flight. IMHO a level 2 safety shall catch a level 1 failure, but if the level 2 has to be checked is not doing the job for what was designed.