G'day, JH. It is not necessary to believe in a God in order to agree with a concept such as the social value in not killing or not stealing (or not doing naughty things to the neighbour's oxen). The atheist is disagreeing with the source, not necessarily the content.
This debate is remote to me (not my Country), but appears to be about alienating 1 group of society. If the the term used in the Pledge had been "under Allah", no doubt some of the posters who are currently promoting the continued inclusion of "under God" would be riled at that as it would be being required them to pledge to something in which they do not believe. I'm guessing that some Atheists feel similarly alienated in having their pledge of allegiance to the USA (which they do not appear to be objecting to) being bound to also believing in a God.
For some people this may be a big issue, for others, not. I was surprised that this was introduced in the 1950's - I had assumed that it was a Founding Fathers deal. Again, from the outside, it is interesting that those who believe that it should not be a big issue for an American Atheist also seem to believe that it would be a big issue for it not to be there.