Ultimately this is a map balancing issue. As a general rule of thumb, if a CT has reasonable cause to believe a prisoner is in vents, it should be reasonable to allow them to pursue it using any reasonable means.
There are many many caveats, pitfalls, technicalities, and definitions that introduce a ton of nuance and need for discretion for this general guideline to be applied fairly. For moments where the CT(s) have no reasonable cause to believe a prisoner is in vents, yes, they should not be pursuing vents regardless from what entrypoint.
On the other hand, if a guard did have reasonable cause to believe a prisoner(s) is/are in vents, then they are allowed to pursue it. I can understand how this may seem a bit in favor of the guards, and frankly, that's kind of the point.
You as a prisoner are not required to go into that vent. Over time you should gain familiarity of the maps that you play, the vents/secrets they have, and the pros, cons, and risks that you take when attempting to rebel. The map maker designed this map in a very intentional way. If they wanted, they could have added a barrier preventing CTs from pursuing from armory- they didn't.
tl;dr get better, don't blindly rush vents and complain when you get domed