Telling users before droplet creation (perhaps some red text) that ssh keys cannot be added after creation would be helpful.
Found out after I created my droplet. Agreed that it can be changed after through command line but if you told me I just had to paste it before creation for DigitalOcean to take care of it for me, I would definitely not manually add the keys.
We specifically do not retain any access to droplets after they are created which is why we can't add SSH keys after creation. We've also added in-line SSH key additions on the create page to make this process easier for customers.