Being susceptible to fascism isn't exactly an American invention. It's a failure of liberalism that leads to this, as democracy is eroded by capitalists.
Yes, the monarchy was reestablished after General Franco's (fascist) Nationalists won the Spanish Civil War against the Republicans and Socialists (and Communists and anarchists, all in their separate groups) back in the 1930's.
It was bad, but mind the cultural context they it was widely considered the "southern" flag (by southern whites) which just happened to also have been used by the Confederacy. There was a century of pro-Confederate propaganda we were raised under, and we've only recently reckoned with that.
A lot of those same people who flew it when they were kids decades ago denounce the thing today, thankfully.
The concern isn't legal, it's medical. Making your own medicine is incredibly dangerous.