I don't know everything about the new policy, but I can say that MIT historically acted quite differently from a lot of universities (or any other institutions). The campus was more open than basically any space. Groups with tenuous ties to MIT held meetings on campus, theater and arts groups used facilities for practice, etc.

In some ways, MIT's history feels like chaos and this openness was part of that chaos. It's a part of the MIT culture that it seems the administration is trying to extinguish. Sometimes, this culture is harmless fun like measuring the Harvard Bridge in Smoots (https://en.wikipedia.org/wiki/Smoot) or how some dorms allow residents a lot of freedom to customize their space (in ways that other universities would go crazy over). Other things were more controversial like Senior House. Depending on who you ask, Senior House was a place of support and community for minorities and marginalized students or a place that had an issue with drugs and alcohol that was responsible for a much lower graduation rate.

I think part of MIT's culture and success was its open and sometimes chaotic nature. It certainly endeared the school many people in the surrounding community who saw it as a place of the community rather than an ivory tower set apart from it. I think it also created an environment where students felt free to try things. Sometimes this was great stuff, but it could also mean students taking on too much or doing things that might not be as great. From what I've heard (and sometimes seen), it looks like the administration wants to push MIT in a more "normal" and controlled direction.

Some people will say that MIT is a private organization that can do as it likes. Legally, they're not wrong. However, MIT gets a huge amount of governmental support - all the grants from the US government for research (which could be directed to publicly-controlled universities instead), all the money to students, all the tax advantages they enjoy, and MIT is one of few private land-grand universities. One could even argue MIT wouldn't have risen to prominence except for the US's funding of defense research at MIT. This occurred because Vannevar Bush (MIT graduate, MIT Vice President, and Dean of the MIT School of Engineering) became head of the Office of Scientific Research and Development during World War II and directed defense research disproportionately to MIT. The government often chooses winners and losers and they chose to boost MIT. As a contrast, when William & Mary in Virginia needed money, the Commonwealth of Virginia conditioned that money on it becoming a public university. Likewise, Rutgers is now a public university despite hundreds of years as a private school. Similarly, sometimes governments pick who can get the government's money without concessions.

Regardless, it's certainly a big shift for the school. Academics from other schools can't just pop by and will need to be escorted to meet with someone, it likely won't present the same atmosphere where alumni in the area would feel like they've retained a deeper connection with the school than most schools' alumni, and it just won't feel like a welcoming space within the city. I'm not going to say that this is the wrong decision because I don't know everything that went into the decision and I don't generally, but it will make things very different. I know MIT has struggled with certain issues on campus, but most of those issues don't seem to come from its open-campus policy as much as internal stuff. I feel like I should have a conclusion, but I guess it just feels sad. Whether they're doing this because they just want MIT to be more normal and controlled or whether they're doing it in response to issues, it's a sad change.