I'm retired now (well, disabled really, but they're basically the same thing), but I was in the leather working machinery business. I will explain my logic further for you. While we had an in house machine shop to make our parts, we couldn't handle production of all of our parts, so we also employed outside machine shops, commonly known as job shops. We would provide the shop with copies of the drawings needed to produce our parts along with the special tooling needed to make them. Some parts didn't require any special tooling, just the drawings, as common machine shop fixtures were all that was needed to make the parts. We were the OEM for the machines we sold, they were our own, we were not reselling another companies product, so we were the place people went when they needed parts for their machines that they bought from us. Now if one of the job shops that we had make parts for us started making more of the parts than we bought from them and started selling them to our customers (or these days sold them on ebay, etc.) that would be totally unethical, as we supplied the drawings to them solely for them to produce parts that we ordered from them, not so we could help setup a competitor.
This is in effect what has happened with Arsenal & NOE. Al went to Jared to have him make moulds for him, he supplied him with drawings of his designs so Jared could make the moulds that he ordered, not so he could setup Jared as a competitor to his business. Now if Arsenal had come out with their own different design mould blocks, with boolit designs that were not from drawings supplied to them by Al from NOE, then that would have been a totally different story, but when they're making a product that looks identical to the moulds they were making for NOE (which were Al's designs, not Jared's) then there's something wrong there, and from the few pictures of the Arsenal Molds I've seen, that's exactly what has happened.
If Al had printed a disclaimer on his drawings that their use was only for producing moulds that he buys from his sub contractors (which is what a job shop is basically), then he would have a legal basis to take Arsenal to court. I don't know if he had such a disclaimer on his drawings or not, but even if he didn't, the use of drawings provided by a customer only for that customers products has been implied by job shops from when they first started.
And as for your question about you going to work for another employer, that would depend on the terms of your employment. Many companies that either train their own employees, or have special processes that are trade secrets have their employees sign contracts that state that if/when they leave the company that either they won't work for a competing company for a set amount of time (5 years is common), or that they won't work in the same industry (again, with a time limitation possibly being set). Now if you had gone to school to learn your trade, and there was nothing 'secret' about what you were doing, then no, there wouldn't be a problem with you leaving one employer to go to another, or even starting your own business. Another example, say the company you're working for gives you a spec sheet for an electronic device they make, and it can't be calibrated without that sheet, and the company doesn't publish the sheet, they keep it internal, but while you have it, you go and make a copy of it, then quit and open up shop calibrating their devices, (and being able to do it better than anyone else outside of the company as you have the spec sheet) would that be ethical?