A smart phone's primary function is to initiate and receive phone calls, or arguably 1/3 of it's primary function if the metric is the Jobs iPhone launch presentation, however since "smart phone" and "iPhone" have "phone" in their names I'm going to argue its their primary function.
People have come to expect that phones nearly always work, and rely on them for critical communication with loved ones, services like emergency services. When these aren't dependable you don't have a phone but instead a toy.
The case made two decades ago is that running arbitrary software on a phone incurs a risk that malware can compromise the device and alter its dependability. _General purpose computers don't have this historical burden._ Phone and mobile OS makers sell their products with their purposeful limitations made fairly clear. You want a mobile device with different capabilities then seek out am alternate device, it's kinda obvious.
There's always communities of people who attempt to repurpose the products they own for purposes the weren't originally intended, and I would like to see that laws that make that hobby more legitimate and legal. I would love to see 3rd parties able to support these hobbyists, that would be great. But Apple, Google with their hardware partners have no obligation to do so, and justifiable positions for making repurposing non-trivial to do.