Well, Wuukiee,
Being a child who was brought up in such a home, maybe I can answer your question (and probably create a few more, I shouldn't wonder).
In my family, it had nothing to do with Halloween being an "evil" holiday. It had to do with Sunday being a special day. LOTS of things weren't done on Sunday, not because they were evil, but specifically to make Sunday special.

A few examples:

No chores were done on Sunday.
We didn't play with friends on Sunday (Sunday was family day).
We didn't watch television, or go out to movies or stuff on Sunday.

The point of all of this was that Sunday was The Lord's Day, and it should be dedicated to Him. If we simply did all the stuff we might do on any other day, how was it then different from any other day?
Just as an aside, if Christmas came on a Sunday, we didn't go to Christmas parties on that day, either. It had nothing to do with Halloween being an "evil" holiday. At least in my house.

Another aside, not to be misconstrued as justification:
One can perhaps relate this type of attitude to that of the Jewish people in the time of the New Testament (and possibly extremely othodox Jews today?). There were very specific laws about what one could and could not do on the Sabbath. Many of them dealt with things that were not considered evil in general, but were simply unacceptable on the Sabbath.

When God said "Remember the Sabbath day to keep it holy," I don't think He meant "take a day off from work to go fishing or watch football once a week."


Footnote: This shouldn't be construed to sound like I'm preaching here, or trying to convince you to all live this way, it's simply meant as an explanation for behavior that some find inexplicable.