Don't take this the wrong way, but that's pretty silly accounting if you ask me. Firstly, I can't think of many people who would consider the return part of a business trip (indefinite or not) their "private" affair and certainly I wouldn't even consider a job under those circumstances. The company/client pays the return fare, if indefinite it'll be on a fully flex ticket so I can move the return date. How or why anyone would ever agree to be "privately" liable for the return portion of a business trip I just can't understand.

Secondly, I don't know of any accounting departments that pick up the tab for other departments. The cost will be coded against whatever department's budget that incurred the travel cost.

I also can't see how this would "improve" the budget. The cash for the purchase disappears from the bank/card account now and therefore you'd see this on the expenditure budget now, not at some indefinite point in future. Bottom lines must agree after all. If it's important that the return money comes out of a future budget, then you'd raise an accrual for it so that the full transaction matches the recorded entry in the current budget, with the accrual then offsetting (part of) the cost and appearing as its own entry on the opposite side of that balance in the current budget (and of course appearing at the debit side of whatever future budget this is being accrued against).

It seems to me that amongst the many reasons for why single journeys are commonly more expensive is that that this means you're more likely to opt for flying twice with the same carrier instead of possibly coming back on another carrier. Ultimately, however, the simple short answer is probably: because the market bears it.