Why is it borderline CRIMINAL to use previate email to conduct White House Business but not when prez tweets about White House Business?

1) Because there were rules in place saying it was bad to use white house e-mail for private correspondence.
2) Because nobody had imagined that anyone might do that, so no such rule exist about it. (I expect that one will be made soon.)

President Obama also used Twitter as President.

What they are worried about is keeping everything documented. 

Sending and receiving government business e-mails using private e-mail servers and clients is not just borderline criminal, it is a criminal act, a violation of Federal law. (Although the President may be exempt from those laws; have to check that . . . ) Government E-mail communications is "point-to-point" (even if it has a lengthy CC list attached) and often carries classified information not to be disbursed to "the Public". For those reasons it needs to stay within the government's "closed" (and highly secured, we hope) network of e-mail servers. That's no different that many large companies. (I worked for a company that encrypted every e-mail through their e-mail clients and servers and then had a second level of security for very "sensitive" information that even the NSA couldn't "crack" quickly.)  

Twitter on the other hand is known to be a "Public Forum" with a multi-point distribution (everyone that cares to look). There is no expectation of privacy, quite the opposite in fact. Items are posted to Twitter with the expectation that many, many people will see and read what's posted. In the case of government officials it's like a press conference without the press "filtering" what's said (until after the fact). 

This post was edited by Salt and Red Pepper at September 26, 2017 9:41 AM MDT

The purpose of using private email is to HIDE something. The purpose of using twitter is to SHOW something. There is no comparison.

The comparison seems to be flawed.

Hilary was using a private computer to store official government communications.

Trump is tweeting opinions.  NOT official government communications.  No different from using any other media outlet to communicate to the American people the "state of the union".  He still has to use official government communication channels for policy changes, etc.