Rallies have various limits on them that prevent them from being farmed for kills, like the “Active battle” restriction, the fact that they are very easily destroyed by pretty much anything, and the lockout after each spawn. Crucially, they are also “optional”. If you think spawning at a given rally point is a bad idea due to nearby threats, you can spawn elsewhere. The normal, fixed spawn points however, don’t work that way.
You have two points, which are in the same place every game on a particular map, which cannot be destroyed, blocked, or locked out. They are there mainly as safety nets. All things going well, players primarily spawn at rallies, but, if the enemy has just forced you back, you can spawn in at your default spawn points, and try to salvage things.
Basically, the difference is that well, no, you can’t really spawn camp at rally points, you can at fixed spawns, save for the spawn protection.
As for what the point of continuing to fight is if there are enemies hanging out around your spawn, I’m going to say that it really doesn’t take much for that situation to emerge, if there is a reason for it to happen in the first place. Newly spawned squads are absurdly vulnerable. Two players with lmg squads in the right place could more or less totally lock down the opposing team on their own. They wouldn’t need to fight their way to the opposing spawn, just sneak down along a well covered path, set up in the spot they hide in every game, and start farming kills. The team that loses in this scenario isn’t necessarily the weaker team at all. They had a push, got wiped out by some well placed HE and a couple assaulters, and someone found their only rally point in the process. While that was happening, the campers set up shop, and now they effectively get no reinforcements for the rest of the game after one of what can easily be a dozen or more pushes in a typical game.
I understand it can be frustrating sometimes to run into spawn protection, especially when you were just defending an area, and suddenly a bunch of immortal guys pop up and kill you, but most of that frustration can be handled by things like tweaking map design (giving more cover to spawns is a good example of such a tweak), and at the end of the day, spawn camping is just too big of a fundamental problem with fps games to let it happen unabated.