Is there a rule among cyclists about intermediate sprints that you can try for points, but not very hard? I see mid-range pushing at these things and I always wonder why at least one rider doesn’t just go all out to get the points.

It’s not an unwritten rule as such, it’s just the practical way to approach racing. The points and prize money awarded at intermediate sprints are minimal compared to those awarded by a stage win, so for many teams and riders there is little point in going all out at the intermediate sprints. The breakaway riders want to win the stage and the sprinters in the peloton want to save their legs for the final sprint to have a better chance of taking maximum points. Or they may be like Cavendish this year and have absolutely zero interest in the points jersey - he only wants to win stages so any use of energy towards a goal other than that is a waste.

Generally you will see a few of the main sprinters having a mock sprint for any leftover intermediate points but it’s mostly about testing their legs and seeing how the other sprinters respond, which is another reason they don’t go all out like they would at the finish line. The prize money associated with winning intermediate sprints or placing higher in the points jersey competition can sometimes be useful for the smaller teams and riders who don’t win as many stages and/or have less funding/lower salaries, which is why you see some lesser sprinters take part but not really try. They are just picking up free points to keep their score ticking along in case they win or place high on a stage.

Here’s some information about the prize money for this year’s race.