Well, it's specifically allowing this because the VAP30UPN fare doesn't have a V inventory restriction, and it delegates into Y, not V, when P is not available. And it's showing up in the search results because it's the cheapest available fare: it's cheaper than a coach ticket on that date.

That used to happen regularly, but it mostly disappeared when UA went to differential-based pricing. But the EWR-LAX fare table is really messed up, at least for that date. You've got the cheapest available fare: VAP30UPN, at $644 + tax, which books into P with no inventory restriction and a 30-day advance purchase. The cheapest available Z fare is YAZ21UPN, at $1094 plus tax and a 21-day advance purchase. And, if you wanted to fly economy on that date... you need to go to MAA0AQEY, at $1124 plus tax. Admittedly, it's refundable, and the others aren't, but that's still a pretty hefty penalty.

So, these fares are written as if they use differential pricing, but they're missing the inventory restriction that makes the differential work. Thus: business class cheaper than economy. (And even if they had the inventory restriction, the YAZ21UPN fare is way out of sequence).