You do a spot check to see if you have a problem. A 50% fail rate is one hell of a problem that warrants triggering a deeper look. God forbid they do a spot check first before doing any other checks on other cars so they know if they need to do further checks.

It is a random spot check but when you have a 50% failrate shouldn't it be investigated further? Imagine going skydiving. There's a parachute spot check that shows 50% of the parachutes don't work and everyone else is given the green light. Would you jump? Somehow I doubt it. The plank check is a similar safety check, except it's done after the race because you can't beforehand verify if the car isn't too low. It's a dangerous sport and safety should be taken seriously.

Also the current approach punishes the driver. It's not the driver's (at least I don't think it is) responsibility to make sure their team gives them a regulation-compliant car. It's the constructors responsibility and the punishment should focus on the constructor, which means at the very least both cars should be checked if one of them fails.