Why ship a low power charger with an overkill thick cable?

1.7A isn't exactly low power. As you note, the several-hundred-watt desk lamps run on 230V, so their current requirements are much less - They could actually be lower ampacity than your charger! For example, this is almost 4x the power levels of USB.

As you probably know, it's the current that determines the size of a cable, not the power. I calculate that your cable could carry up to about 8 amps, which is a little more than 4x the rated capacity of your charger, which may put out higher amperage under short-circuit conditions. In a short circuit condition, you'll want a fuse or PTC in the charger to cut out, rather than burning the cable.

It's not really overkill.

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.

Two things come immediately to mind - the first is that larger diameter cables give less voltage drop. I've got some really crummy USB cables, for instance, which have enough voltage drop that my USB powered HDs won't work with them. There may be some reason that they wanted to keep the voltage up, and therefore they went with heaver-than-you-expect cable.

The other thing is that cable (like many other things) costs less in bulk. They may well have decided that using the same cable for lots of things is cheaper than designing specific cables for every item. This might produce some overly bulky cables at times, but if they can manage to buy 2 miles of one style cable per month instead of 1 mile of two different cables, there's likely to be some cost savings there. If nothing else, it's almost always a savings if the factory doesn't have to handle two different parts.