Easy answer, as accessible as the coach wants them to be.
Well, actually, that answer is likely better stated in a different way. Access to practice is directly related to what a coach can get away with, in most cases. Meaning, the more access a coach can cut off, the more he will.
Access is not exactly fun for coaches. Thus, while fans and media types love being able to attend practice, get a look at drills and see scrimmages, coaches aren't looking for extra eyeballs on their team.
They don't need the local radio folks diagnosing spring problems from a distance. They don't want the message boards attempting to make depth-chart decisions for them. They don't need the headache of having to answer questions about the team's bad practice because the staff was trying something new.
However, circumstances often dictate just how hard of a stance coaches get to take. Curmudgeonly coaches who keep winning get to keep closing practices, because they have earned that ability to control the message. Other coaches, who need to build goodwill with the fanbase, are forced to be more inviting, as a way of letting fans know it is "their" team, too.
Let's face it, flat rules and policies governing how open a team is with its spring ball all sound really nice, especially to those tasked with covering it and the fans who hang on every morsel of information. Everyone wins, except for the actual people involved in making the process of spring ball go.
Access is granted or denied on a case-by-case basis, whether the comparison is Oregon versus North Carolina or Arkansas versus Tennessee. The Ducks have had closed spring practice sessions for a few years, and while folks would love to see inside the sessions, Oregon is in a spot where it does not have to allow it.
In the case of Arkansas versus Tennessee, you have a Hogs fanbase that is happy to have an actual coach after the mess of 2012. That enthusiasm affords their new guy plenty of leeway. Bret Bielema closed spring practices, justifying it by saying the team need to come together and he needed to evaluate his athletes, and it was all OK.
On the flip side, Butch Jones at Tennessee is dealing with a fanbase that is famished and hungers for positive football news. Jones is obliging in a big way, giving media access this spring and doing all the interviews he can to help get the Vols' fans back on board, supporting his team.