I might be wrong, but I think the Watchtowers rejection of the traditional cross comes more from Rutherford(?) trying to make his religion "differant" and finding ways to distance themselves from the more mainstream churches. One only has to look at many of the stances taken on things like Holidays, voting, blood, ect. to see that these are used as things that make the Witnesses "differant" and thus correct.
They get offended mainly because they feel they are correct in their view, and the one speaking of the particular doctrine in question is ignorant of the facts, or simply just ignorant of spiritual things in general.
Bingo, give the man a cigar.