Is it because it then becomes the sale of a business in their eyes, rather than just a residential property perhaps?
When we bought (in Scotland), our solicitor did charge more than for a standard sale. However, that's because the age and condition of the house, and the need to verify boundaries made more work for them (we only bought half of what the vendor owned, so the land ownership records had to be split up). There wasn't anything complex because it was a smallholding though, and we did the transfer of the CPH ourselves.