Although it's very much anecdotical, I, too, distinctly remember that Google defaulted to AND (unlike today, a very hard AND at the time), and that this made a noticeable difference in searching habits.

My theory on why this might matter even for people who knew how to use the operators is this: With OR as default, you would first try your query without operators, get page upon page of irrelevant results, and then start to narrow your query down. With AND as the default, you would type in the query, and if you only got a few irrelevant results, or often no results at all, you would try alternative terms instead.

It seems that progressing from no result to desired results by choosing alternative terms just makes more sense than having to wade through irrelevant stuff, and the default encourages one methodology over the other.

Today, it's very hard to make Google return no results at all. Not just because the amount of content grew to an unimaginable scale in the meantime, but also because Google has become way, way fuzzier in the way it interprets search terms, likely to better suit a larger and different audience. A lot of times today, I have to switch to "verbatim" mode first, at least for technical stuff.