I feel bad for your ordeal but you could have avoided all of that trouble by including your own licensed generic looking icons to represent the computer you were connecting to. Instead, you chose to stubbornly use the defence that just because an icon is available to you via an API on a mac, that gives you the right to use it any way you see fit. What if the customer was running a custom replacement set of icons on their desktop? How would your app be able to tell the difference between the original icons and the replacement? What if the author of the set of icons only gave permission for use on mac desktops and not for any other use? Your program would cause the end user to be breach of the distribution agreement without their concept or knowledge.

As a windows developer, I can access virtually any icon on a windows box through various APIs but that does not give me automatic permission to publish those icons on a website or within a a separate mobile client UI as images.