A company can simply decide to pay the carbon tax, though.

Actual penalties mean that you are actually risking your company if you don't comply. Fines are different if the folks in charge have to pay some as well, for example, and/or if they are a significant portion of profit (percentage of profit is a harder hit than a flat carbon tax that you can plan for if you would like). After fines? Jail times for those in charge and closing/auctioning off your business.

Carbon taxed don't do these things, and I'm not a believer in the market taking care of this since carbon taxed don't actually make anyone get more efficient than their competitors, who are only likely to get efficient if it is profitable. Carbon taxes themselves aren't likely to do this.