From Steven Levy's Hackers page 228:
The Homebrew Computer Club crowd was out in force when the [MITS] Caravan met at the Rickeys Hyatt on El Camino Real in Palo Alto in early June [1975], and were amazed when the found that the Altair on display was running BASIC. It was connected to a teletype which had a paper-tape reader, and once it was loaded anyone coudl type in commands and get responses instantly. It looked like a godsend to those hackers who had already sent in several hundred dollars to MITS and were impatiently waiting for BASIC....Years later, Steve Dompier tactfully described what happened next: "Somebody, I don't think anyone figured out who, borrowed one of their paper tapes lying on the floor." The paper tape in question held the current version of Altair BASIC written by Bill Gates and Paul Allen. Dan Sokol later recalled that vague "someone" coming up to him and, noting that Sokol worked for one of the semiconductor firms, asking if he had anyway of duplicating paper tapes....Sokol took the tape to his employer's, sat down at a PDP-11, and threaded in the tape. He ran it all night, churning out tapes, and at the next Homebrew Computer Club meeting he came with a box of tapes.
So the HCC members first got their hands on Altair BASIC after Micro-Soft had already written it. Distribution of the stolen software was not limited to those that had already ordered it from MITS.
There are lots of things you can bash Microsoft and Gates about, but Altair BASIC isn't one of them. You don't need to lie to make your point that Gates is a snake.
You want to pretend that nothing ever happened before that.
One last time: Bill Gates was using code "borrowed" from other people to create the BASIC compiler in the first place.
So when his code started getting passed around, no one thought anything of it. After all, they were all people who had contributed code in the past. Code was treated like a library book. You take it, you give it back.
Bill Gates took code, and then wanted to sell it back. Most people using hobby computers thought this was just fine, if a bit silly. No one paid for code, it came with the computer. If you wrote something, you passed it around.
Bill Gates got his start by selling software that was based, at least partly, on other people's code.
The very group of people that contributed that code is the one he attacked with that letter. He's hated the hobbyist scene ever since.