Not so fast.
The Church discourages capital punishment if the particular state or nation has the resources and the facilities to keep the perp jailed for life.
Now in the case of burning a heretic, it just so happens that the Church does NOT have the facilities to keep the so-and-so jailed for life.
QED: Bonfire Time!
As I understand it has been about 110 years since last heretic was executed; IIRC, the Bishop of Seville was burned.
That makes it high time for another public example, just to keep the threat credible.
"Marshmallow-on-a-stick? Sausage on-a-bun? Cheapest prices for best treats, even if it's like cutting me own throat!"