Why is toilet paper noticeably easier to tear when it hangs over the front?

Your torque analysis is right and it is not the whole system. What differs is where the paper leaves the roll and how far your hand is from that point.

In the over orientation the sheet leaves the roll at the top and comes towards you. You can grip it close to the roll and pull almost straight down, and there is very little free paper between your hand and the point where the sheet is still supported.

In the under orientation the sheet leaves at the bottom, behind the roll, and hangs against the wall. Your hand is necessarily further from the departure point, and the paper between them is unsupported and free to stretch.

That matters because tearing depends on applying a sharp local stress, not on average force. A long unsupported length absorbs your pull by stretching and by the roll starting to turn, so the force arrives at the perforation gradually. A short length transmits the jerk directly.

The second factor is that the two orientations differ in which way the roll wants to rotate against friction, and this is measurable.

A roll on a spindle has some friction, and in one orientation the paper's departure geometry means your pull also presses the roll against the holder slightly differently. Combined with the paper's own curl — the sheet has a memory of being wound, and in one direction it wants to lie flat against the roll — you get different amounts of resistance to unspooling.

Higher resistance to unspooling is what you want when tearing. You are trying to hold the paper still while snapping it, so anything stopping the roll from simply spinning and paying out more paper helps.

Which is also why the whole question is really about one-handed tearing. With two hands, one holding the roll, the difference nearly vanishes.

Easy experiment if you want to check the reasoning rather than take it: hold the roll still with your other hand and tear in both orientations.

If the difference largely disappears, the effect is about roll rotation and unsupported length, which is what the answers above claim. If it persists, something about the perforation geometry or paper curl is involved instead.

In my own version of this the difference did nearly vanish, which I found more convincing than any explanation. It also suggests the practical fix for anyone stuck with the other orientation: a holder with more friction, or one that presses the roll, gets most of the benefit back.

Worth mentioning the non-physics reasons, since they explain why the argument never ends.

Over is generally easier to see the end of and to grab, and it keeps the paper off the wall — which matters if the wall is textured or dusty. Under is harder for cats and small children to unspool onto the floor, which is a genuine and much-loved argument in some households.

So the physics favours one, and the household constraints occasionally favour the other, which is exactly the situation in which people argue forever.