Slavery is immoral because it takes away freedom of movement (and is thus a form of imprisonment), not freedom to create contracts. There are no fundamentally fixed moral rights relating to trade because there is no universal objective notion of property. Property is an entirely social concept and can only exist through social contract. You only "own" an object to the extent that everyone else agrees you do.

Most people want there to be regulation of business, even if some of it is corrupt and inefficient. Most people want governments to collect taxes and control shared resources, even if this is inefficient and wasteful. These things are seen as a necessary evil. If it's what the public want, then it's not immoral, because the entire notion of property (and by extension trade) only exists as a social contract. To go into detail for your taxi example, the roads were built as part of a social contract where people pay taxes under the expectation that the government will execute some form of transport policy. Tax payers would have anticipated some form of taxi regulation to occur over future roads. For most people this is desirable because they want to be protected as consumers. If you take away taxi regulation, then you are not honouring the implicit contract that allowed for the creation of the roads.

You may personally disagree with this widespread desire for regulation. It is also reasonable to say that corrupt and inefficient regulation is immoral. However I think it is quite dangerous to argue that regulation itself is fundamentally immoral. The history of the industrial revolution shows what kinds of contracts ordinary people will willingly subject themselves to if they feel they have to.