Great question/dilemma. Let me give this a go.
INT: (Add other situations below)
1. On B at home plate during steal attempt
2. On B/R on play at the plate
3. On B/R outside running/lane to first
4. On R during DP attempt (see FPSR vs FSPR???)
5. On R touching any base, except IFR
6. On R between bases (ala Jackson, A-Rod)
MC: "The umpire's judgement" is just that, no further explanation needed. I recommend reading the following thread on MC; Erstad - Estrada Collision at the plate by CoachJM.
It boils down to FPSR vs FSPR???
1. The actual rule is the Force Play Slide Rule. It protects the defense from injury only during a simple force play. A great rule, and easier to apply.
2. There is NO Forced Slide Play Rule when a tag is required. However, due to factors such as sportsmanship, injuries, hospital bills, etc.; Rule Committees now emphasize a MC rule on any TAG PLAY. A great rule, but much more difficult to apply.
Why the difference in penalties:
1. INT is just that, penalize it by rule.
2. MC is totally a judgment call. Think about the fielder who drops/misses the ball completely before applying the tag during a slide play. How does an umpire decide whether a proper slide or MC jars the ball loose? Will you impede a runners progress because of poor defensive skills? To borrow the famous line, "If INT doesn't fit, then you must acquit (:MC
."
Play BALL! Play FAIR!