Understanding capacitive load conditions for a microcontroller
The first table clearly says "Load Conditions for Device Timing Specifications". That means that the rise and fall times specified in other areas of the datasheet (typical 5ns maximum 25ns for one I looked at) are to be measured with that load. It's not a maximum per se, but if your load capacitance exceeds the 15pF you can't complain if a port pin takes 26ns to rise/fall- which most likely would happen at a temperature extreme. 

Table 32-15 is a bit different-it indicates the maximum load on the pins in question for the part to function properly. You should not exceed a 50pF load directly (no series resistance) on the port pins, nor on the OSC2 output. The 15pF maximum load on OSC2 when an external oscillator input is used is because that output is also the input to the clock of the microcontroller, so if one loads it excessively it could slow the edges down enough to compromise operation of the micro. The 400pF loading on the I2C pins indicates it conforms to the NXP (née Philips) spec for unbuffered 400k I2C.