The 'get incorporated' trend has certainly been more noticeable in the past couple of years.
Generally speaking I do not give course to such employment offers, in other the tax rate for incorporated individuals/small business is too high.
In regards to thw reasoning behind this, I believe it's mostly a question of company culture and perhaps the expenses the employer needs to pay (gov tax, etc).
A bit off-topic but a while ago someone posted a similar tool here on HN that allows one to produce diagrams - they looked pretty much hand-drawn. Does anyone have a link?