[mbyron]

MLB (and MiLB too) has no malicious contact rule. NCAA and FED do, so they're different.

OBR thus does not prohibit crashing any fielder at any base. You're right to observe that you don't see it at other bases: at 1B, of course, it wouldn't make sense.

At 2B and 3B you don't see it because crashing a fielder doesn't make baserunning sense: on a force play it does no good, and on a tag play you're likely to end up off the base and tagged out anyway for the trouble.

It's also against baserunning tradition, and as you point out there's an unwritten rule against it. That's a rule that the players enforce among themselves: the umpires do not enforce it.

The existence of unwritten rules and enforcement (usually a fastball in the ribs) is part of what led amateur ball to add a MC rule including a penalty enforce by umpires.