Because the intent of such a hypothetical law would be to reduce/capture C02, but the end result of it is that it financially incentivizes generating more C02 through some unexpended/unanticipated means.

An unintended consequence might be that yes companies do correctly follow the law and adding carbon to concrete is successful, but also adding carbon to concrete in high enough volumes makes the lifetime of the concrete lesser when exposed to rainwater for long enough, or something like that, resulting in the need to replace the concrete more frequently.

Its the difference between people acting in good faith vs neutral/bad faith with respect to the intent of the law/regulation/etc..