As for cars people overpay for poorly built cars all the time thinking they are better than cheaper cars by more reliable manufacturers. I guess they carry their habits over into punting but like you say reassuringly expensive doesnt apply here or ever really IMO.
No they don't. These people do it in punting solely as a means to appease the girl thinking it will make them more liked by the girl saying this.
Usually when they suggest the girl puts her prices up. As soon as the girl agrees, the next question from them usually is, "because I was the one who suggested it, will you keep me at the original rate".
For those that do this. If you think the girl is worth extra. Why not just turn up with an extra £20 and you pay her the extra each time you see her. You will curry that favor you wanted from her for doing that. But I also promise you, after one or two times, you will get sick of doing that. So given that imagine how everyone else feels due to your bastardy trick!