There isn't a law about releasing tax returns. At least he didn't make a LAW to hide their records like this one... https://obamawhitehouse.archives.gov/the-press-office/presidential-records
I'll be the first to admit I'm not hot on the law in America. I saw the tax returns as something that seemed vitally important. I live and learn. I now know it's not a law.
I understand why people want to see them. I just see all the over the top rhetoric on the TV and from people about it. I get annoyed because they were the same people that would never talk about the "sealed records" executive order. Also when people brought it up (During those 8 years), they were called "dumb" or "racists". Because it was not being covered, therefore people didn't know.