Removing the amount of choices in front of people allows people to feel more confident about theirs.

If you give them the option for 4Gb, 8Gb, 16Gb and 32Gb instead of limiting to 16Gb lots of people will freeze and never make a choice or get frustrated when they see the markup for 32Gb and avoid buying altogether because of that.

In limiting the amount of upgradable RAM on their sales page they show you choices that are most likely going to be easy to sell.

They probably figure it won't necessarily work very well, so people would complain, so they'd rather just short-sell their machines.

Apple simply does not cater to the crowd that does aftermarket upgrades to their computers. The limits they list on the box are usually the top configurations they will sell you. They do not bother testing and certifying every combination of third party component that might happen to fit into the machine and work.