Am I "working" in Spain?

I suppose you work at home?

I don't see why this should be wrong but I say immediately that I have no knowledge of the Spanish law at all. Just my common sense.

Thanks, Primadonna. Yes, I am doing this from my "piso", so it is not a "job". This, then, could apply to others who --  let's say -- have a foreign contract (e.g., programming) that they can fulfill from anywhere in the world. They don't have a job here, and the money is not being "earned" here. They are just here to spend some money, which is my case. Of course, everybody "hears" what they want to hear.

Technically any income earned no matter how small or out of the country is supposed to be declared to the tax man even if it meets US or Spain's tax free thresholds.
There are a number of laws pertaining to tax on foreign income depending on a number of things but realistically that shouldn't concern you as you shouldn't be earning a thing while you are here.

You are deemed a fiscal/tax resident of Spain the second you spend 183 days here so from that moment on then you need to file a tax return and whether or not you will need to pay tax depends on double tax treaties and various thresholds, however I don't think you have to on a non lucrative visa(or should have nothing to declare at least) so really you have a moral conundrum.

Just to clarify a bit further, from all the reading I have done it is the physical act of working from your flat that is the sticky part.
You can live off of savings or income earned outside of Spain but by the letter of the law any time you conduct any business on Spanish soil that is regarded as working.
It seems stupid and probably is but I wouldn't go running off to tell Hacienda about it.