Because they had such cables in stock at the moment? Maybe they were able to buy them cheaply. Some manufacturer may have overproduced them and was looking to get rid of the stock or they were designed for another product that turned out to be a failure. They may have same power supply case for several products and just change the electronics inside. Some of them may require more power than others.
Also note that 300 V insulation isn't that much. It could be more expensive to look for cable with lower insulation ratings. For example ALL professionally made serial port cables I have have 300 V insulation and we all know that 300 V isn't anywhere near the voltages used in RS-232 ports.
Another point could be the bulkiness of the cable itself. While the cable may be overkill for electricity transmission, consider usual environment in which the cable would be used. You can't assume that the user of the cable will be able to handle the thin cable well and that the environment may will be friendly to thin cables. Remember there are pets that may want to eat the cables, someone may trip over the cable, the cable may be bent many times during it's lifetime and so on. In such cases it may be better to provide a good quality cable and just using one which is thicker may be the cheapest solution.
Another point is the appearance of the product itself. You didn't mention the exact camera model, so I can't dig up any information about it, but sometimes the sufficient cable may look and feel cheap or weak and that may leave bad overall impression of the product. It's certainly cheaper to just put in a good quality cable in order to impress the customers a bit more.
So to sum this up: I don't see a valid reason to use this cable from the point of view of physics, but keep in mind that engineering is applied physics and that makes the business side of the problem important to engineers.