In my town, there are a few people that try every year to get a nativity scene
displayed on the courthouse
lawn. This is the excuse
One argument they made is that the Supreme Court officially acknowledged that Jesus is the reason for Christmas. This makes perfect sense, since the holiday is intended as a celebration of his birth (even if it is celebrated at the wrong time; there are those that argue the position of the shepherds indicates summer, and others say that if Mary and Joseph were heading to Bethlehem for the New Year's Day census, then it must have been spring because the Roman New Year was the spring equinox).
That aside, winter does not equate to Christmas, and therefore it is inaccurate to say "Jesus is the reason for the season." Winter occurs because the angle of sunlight is such that part of the earth has shorter days, which causes the temperature to be cooler.
Most importantly, Christmas is not the only holiday celebrated in winter. Jews have Hanukah, African-Americans have Kwanzaa, and pagans have the winter solstice itself. Thus, to avoid discrimination, if the county allowed the nativity scene it would also have to allow a Yule tree, a menorah, and whatever else anyone offered up to represent a holiday in December (or, really, any other time). Fortunately, the county commissioners realize this (except for the one who got fired in 1999, partially over this issue) and keep denying the request that the nativity scene be displayed.