California

The government, both state and federal, use incentives to encourage certain behaviors. Tax breaks for retirement spending, education, child bearing, solar installations, EV purchases etc are all examples of this.

During the EV adoption phase the govt will incentivize use of EVs through various means such as tax breaks, preferred parking, preferred lanes (carpool lanes).

Once adoption rates are high enough, the incentives slow down. It's a good thing to initially give reasons to adopt tech or behaviors that benefit the common good.

It can be argued that it might be time to remove carpool access for EVs for several reasons, and it can be argued that we should keep them for now. I don't have a specific opinion on the matter of fact, I think both perspectives are reasonable (as you stated carpool lanes are meant to encourage car pooling, which has environmental and traffic benefits).

EVs only have an environmental benefit in this case so once we reach a tipping point where the environmental benefit is outweighed by the detriment to traffic flow, it's time to revert the incentive. I don't know if it's time yet, I'm not a traffic/environmental analyst.

But I think this change has more to do with the administration's battle against EVs and green tech in general than a calculated cost benefit analysis.