Well, that's total bullshit. Bush did join the Tx ANG but he also went to flight school and did learn and qualify on the F-102 "Duce" one of the hardest aircraft to operate in the USAF inventory. He did offer to go to Vietnam when that type was deployed there, but his flight hours were low compared to many other pilots--who had often flown active duty and been in the ANG for years--so he wasn't accepted for that.
Then, towards the end of his service, the F-102 was being phased out so flying time became increasingly restricted. As is typical with such things, members, both officer and enlisted, had more flexibility about where to do their duty and like others, Bush took advantage of that to choose duty that suited him.
Nothing special or unique in any of that, and don't even bring up those bogus memos that floated around, you know, the ones that ended Dan Rather's career due to them being forgeries.