Whats the difference between quackery in corona times vs before? Either you protect gullible people all the time, or you consider them responsible people all the time. Only thing that has changed is the scale of the fraud.
The difference is the extra attention he's brought on himself. He was just a background scammer before, now he's inserted himself into an ongoing global crisis. It's the difference between annoying someone when they're in a good mood vs annoying them when they're in a bad mood and scared. Institutions are only as rational as the humans running them.
The department of justice has limited resources. It's impossible to go after literally every case of fraud, "all the time". Law enforcement is always picking and choosing what to target, and this guy advertising a coronavirus cure at this current time is like shooting off a flair in the dark asking to be noticed.