Refusing to pray on the ground

I have encountered able-bodied people who choose to pray sitting — because they have no prayer mat when out and about, and the ground is dirty.

I asked the person who did this most recently, why not pray standing, properly? He said it's dirty; but it's carpeted and indoors and even a corner where no one goes. He shrugged and prayed sitting.

But that got me thinking, Islamically the place is visibly clean, and smelling it or any other sense makes the ground in question clean.

The guys qualm is that, it could be dirty — if it could be, it isn't. The principle in the shariah, as I have noticed, is: if in doubt, pick the more positive option.

The problem here wasn't the ground and it's impurity; it was the person's false sense of hygiene which prevented the salah from being established properly.