If you hired me to build your house for $100k, and you paid me, and then you sold the house, making a profit of $300k, would you give me $150k? Likely not.
I think a better argument is below from 4ntonius8lock, who says that employees were supposed to get (a) $100k AND (b) stock of some value. But employees were not told all the rules under which (b) could be zero.
Obviously works for hire do not always imply joint ownership. The reason for the discussion is that we are talking about startups built with implied joint ownership. We are trying to infer the equitable joint split for the reward in building something that is implied, often overtly, to be owned by both parties. Both parties in the two person startup are investors.
Most of the things we purchase with money (we spend time to acquire someone else's effort), are sold as someone else's time. If you pay someone to build a house, it's because they were offering that time for sale. This is why software consultants at most startups never get any equity, because their time was already on sale, and was being auctioned off.
We humans are such primates, though, that we regard those with far more money as essentially higher class in the social hierarchy, and venerate their actions, and their property, as inherently more valuable than someone with less.
So when investing in a company with money, you are able to participate in that company by buying someone else's time. But, assuming a fair exchange (which is usually not the case for the laborer), then the one who works is trading one year of time for one year of time from the investor, in the form of dollars (assuming a 100k/year salary). However, the investor isn't trading their time in the form of calendar time, but in dollars. The worker is trading their time not in dollars, but in calendar time. The point is, it's an equal exchange. Which, if there are two people, then both would split the reward 50/50 assuming all else is equal.
We get confused as primates because we are comparing dollars and time, not realizing that we need to convert to common units. To know the true fair split, we have to know how much each person invested in the same units, time or dollars, but not both. We have to convert all dollars invested to time, or all time invested to dollars, in order to know the true investment ratios. In the example of Person A and B, both put in 1 year of time, hence a 50/50 split.