There are several reasons for this:
1. Some people like to smear art in shit, it makes them feel good about themselves.
2. Its a job! Some poor sap was jobless and then some guy came up to him and offered to pay him to draw on his toilet paper.
3. Some pople will stop at nothing to pamper their ass that little bit more. Be it a fifth ply or a cute little dolphin drawing.
4. The Richard Nixon scandal "watergate" was infact started by Nixons complaints about insufficiently drawn on toilet paper in the white house, ever since then to preven any sutch thing happening again, toilet paper companies have invested billions of dollars in overseas oil companies so that george Bush could wipe his a.ss with 20 ply toilet paper with gold frills, and then I woke up and realised it was all a dream
The end.