Say John make a platform right now and set the rule that there's no negotiations permitted. Builders accepting the proposal must accept at the listed price. Obviously there remains a variation in the quality of the different proposals (some builders may be more polite, or use better wood). Obviously, the key difference here is that I would be able to choose what builder I want, but the customer on Uber doesn't get to decide that. So let's place one more restriction: the platform decides which builder gets the job behind the scenes.
Now, would you say that builder is employed by this website? If so, at what point did this turn from self-employment to employment?
> Similar argument for changing toppings on a cake - a caterer can negotiate changes to the cake - an Uber driver has no such flexibility.
If the customer wants a cake for wedding: vanilla with 3 layers and a cherry on top it seems a bit inappropriate to contact them saying "I can't make a vanilla cake but how about chocolate?"
It's a limitation of the platform (limitations made to improve customer experience - imagine if you had to negotiate for every taxi ride, you don't do this for local companies). I'd argue that the hypothetical platform described can set any number of limitations, as long as the builder can hop off and get his work elsewhere at any point.
Really, imo, the key here is how much control the Uber driver has to refuse tasks, and if they can seek the same work elsewhere (eg is Lyft available in the city?). You wouldn't tell your boss "eyy sorry chief, bit tired, gonna head home, give the fare to someone else, byeee!! hangs up" -- but you can on Uber.