By the way, amazon.com used to use some algorithm to make things less expensive for a new customer, and start raising prices to the level they found they could get the most out of the customer. Price discrimination however can get your customers really pissed if they found out, so the only price discrimination companies do is very subtle things that nobody -- other than economists -- can really see beneath that what they are really doing is to maximize profits: things like cheaper tickets for kids, students and elders (while of course, making them more expensive for people that work). Walt Disney parks make the tickets cheaper for the locals, and more expensive for tourists, since the locals are more sensible to it. Executive class. Etc.
With regard to cinemas, one extra problem is competition. When people decide what movie to go to, those cinemas that practice price discrimination on the movies, may lose customers because they rather go somewhere where they already can expect how much they are going to pay.
Also when you do some price variation -- for how minor it may be -- your customers will get sensible to it. Instead of just deciding a few songs they like, they'll also have to take price in consideration as well, and that could reduce their enjoyment in the service. People don't like to have prices in their face, especially in entertainment. Businesses have software and economists that interpret their statistics to make sure they are doing the best for *them*, which means whatever makes the customer happy, because they want him to come back, and spend more money in them. ;)
Some cinemas here actually sell inscriptions, so you can go to whatever movie you want, how many times you want to. I have been told of an online music retailers that instead of selling music, you pay a monthly inscription, and then you can stream whatever you want.
I don't know why cinemas don't try to do price discrimination a bit different through discounts though... For instance, they could partern with stores for discounts on less popular movies. Maybe it's because those movies are too short lives for a discount scheme. Or it could be because they don't want to try to pressure people into seeing a movie that they'll likely dislike and then blame the cinema for the experience... (whether it is conscious or not, it's important people have a good image of your company, otherwise they will tend to go somewhere else, next time they are making plans.)
Something that should be noticed as well, is that in movies, the publisher has a lot of control over the retailer. There is very little elasticity here. Many times, people go to your cinema, because they want to see some new movie, and you aren't going to convince them to go watch something else because you couldn't get an agreement with the publisher. You not only lose customers this time, but in future too. So I think publishers pretty much set the prices in this business. (I don't think this is that true for music retail though because a lot of times, like iTunes, the retailer service is couples with the media player, so there is an extra price, in the form of effort, to check some other store...)
Equality before the law and material equality are not only different but are in conflict with each other; and we can achieve either one or the other, but not both at the same time. -- F. A. Hayek in The Constitution of Liberty