For 16 years? Seems a bit...harsh, no?

Not in the specific instance you're mentioning, no.

The big "sentence enhancer" for Martinez was that he had prior convictions. Under Iowa law, if you have two prior felony convictions you get charged as "habitual offender" and your max sentence can go up to 15 years on conviction regardless of what the sentence would otherwise be for the crime your charged with. Three strikes and you're out, you know.

He went out of his way to show that he deserved to be treated that way. He threatened further reprisals against the church whose flag he burned, allegedly threatening to burn down the church. He was unrepentant and presented himself in court as someone who would absolutely keep committing these crimes. He did everything he could to demonstrate to the prosecutor and the court that he was exactly the sort of person the "habitual offender" provision was directed at: unrepentant for the crimes he had committed and perfectly willing to repeat them:

Story County Attorney Jessica Reynolds told KCCI since Martinez has a long history of harassment and was also charged as a habitual offender, he faced a harsher penalty.

“I believe him to be very dangerous,” Reynolds said. “That’s why my office recommended the maximum sentence.”

cnn.com - Iowa man burns lgbt flag church trnd

So the thing that drove up his sentence was the "habitual offender" enhancement. Without that, he faces a three year sentence max. But because he's got prior convictions for drug and theft crimes, the Iowa penal code ups the max penalty for the arson count to 15 years. Pretty standard law and order proposal, usually offered by conservatives - two or more prior felony convictions gets you a 15 year stretch if you commit a third one. When a guy has shown himself to be a dangerous criminal by having those kinds of priors, he gets the book thrown at him if he commits a third crime.

In other words, he acted exactly like the sort of criminal defendant that law and order Republicans are appalled by when some "liberal" district attorney doesn't throw the book at them, and then later they go out and kill someone. Everything he's done at that point indicates that he's a danger to this community. The County Attorney and the judge agreed. His behavior since then doesn't exactly indicate they were wrong:

thegazette.com - Ames flag burner charged with indecent exposure for story county jail incidents

Usually conservatives are better at picking the right person to aim their 80/20 issues at. A druggie thief who's committing violent crimes against churches after building up multiple felony convictions, who gets put away under the sort of law that conservative law and order folks love? That's an odd choice....