Copyright statement (US Government) in acknowledgments - what to do as a reviewer?

The article (i.e. its contents and references) are within your remit as a reviewer.

Checking copyright details and investigating any the legal ramifications will not be relevant to the review you should be doing.

You can mention it to the editor in your report, but you do not need to. Your job is to evaluate the correctness and other merits of the manuscript's contents, not to deal with issues such as copyrights. Those are the job of the editorial staff. Moreover, the editors and publisher are probably experienced in dealing with the copyright requirements of government employees and should know to be on the lookout for situations like this.

If you want to mention it, I would suggest saying something like:

I noticed an statement about copyright in the acknowledgements, something that I had never seen before. I just wanted to bring this to your attention so it does not get missed.

I think it is pretty unlikely that the editor isn't already aware of this, but, yes, you should mention it somewhere in your report to the editor. It amounts to a restriction on what the publisher can claim as "reserved rights".

It says, in effect, that if the publisher, accepts this article (and obtains its copyright) the publisher must thereby grant the U.S. Government a license of a certain sort. This means a conditional transfer of copyright.

My guess is that this is fairly standard for work produced by the government and in some cases, work produced by others on government grants. The government doesn't want to create a work and then have to pay to use it internally (i.e. for U.S. Government purposes).