If I build an operating system + hardware, should I be legally mandated to provide tooling to make it easy for users to find and install arbitrary software?
Take the world "arbitrary" and we can see the truth.
Now let me rephrase this in the right way, to avoid the language tricks used on your question.
Should the user have the right to install software on the device they bough and therefore, they own?
Its not "legally mandating" the provider to be "forced" to do anything. Its just a matter of rights of the user who buy stuff from anyone.
Its a pure ethical thing to do, that protects the user freedom, and thats what the law should do, to protect the rights of the party with the least power in a negotiation between two parties. Especially if the concept of property and ownership is involved.
I dont get the half of the comments in this thread. If we depended on your point of views, we would be stuck with the Intels, Microsoft's, Apples and the likes with absolute control into our digital lifes.
Remember Prism and the collusion of goverment spy agencies and tech companies to know everything everyone was doing?
Now imagine how much power this represents? Imagine a modern fascist with that much power on his hands? What would he would be able to do?
You dont even need to get into conspiracy theories, but just think about the rights and freedoms of the users against the effort of tech companies to control them.
But to avoid the centralization of power and control would be a side-effect of people standing for those simple right's.