Any decent immigration attorney who works with tech companies has a virtually unlimited stream of work available. I very much doubt this is the reason for the posts.
Seems unlikely since lawyers have a notoriously high un/underemployment rate. Unless we are using circular reason to define "decent" as anyone with unlimited demand.
“All lawyers” is a meaninglessly broad group to compare to “experienced immigration lawyers with strong connections in an industry that requires a lot of immigration lawyers”.
No one is asking you to read them. Feel free to ignore it. I don’t see anything wrong with YC’c immigration attorney doing an AMA on HN which is hosted by YC.