Why can I enable a hotspot on Android 5 but not Android 9 with same sim card? I have two phones, an old Nexus 5 running Android 6 and my new-ish Pixel 2 running Android 9. I have only one sim card (Cricket). I can enable a hotspot on my Nexus but not my Pixel 2, which says "To enable, go to att.com/mywireless or dial 611".  Obviously, I didn't expect to lose hotspot services when I upgraded.

From these observations, I infer that Android 9 (but not 6) enables carriers to implement policies regarding mobile hotspots. Is that correct?  Is there a difference in behavior between Pixel 2 phones bought though a carrier and those bought directly from Google?

And, if Android does facilitate device limitations, does anyone have backstory about why Google enables policies that are counter to anti-net neutrality?

Excellent relevant background: How can phone companies detect tethering (incl. Wifi hotspot)
From these observations, I infer that Android 9 (but not 6) enables
  carriers to implement policies regarding mobile hotspots. Is that
  correct?



Yes  



  Is there a difference in behavior between Pixel 2 phones bought though
  a carrier and those bought directly from Google ( Device Manufacturer )?   



Yes    



  If Android does facilitate device limitations, does anyone have
  backstory about why Google enables policies that are counter to
  anti-net neutrality?



Yes    





Many Carriers wanted control over tethering so Android just did what was wanted in that aspect, some carriers wanted to charge extra for tethering and even offer tethering specific deals that allowed you to get all your devices online for the cost of a single connection, some carriers offered unlimited data connections with mobile internet, thinking that it would only be used for everyday things like YouTube and Facebook, not expecting people to run WiFi hotspots off of a single device and effectively give internet access to anyone in range for free... Theirs many reasons to restrict network sharing, for example... let's say you purchased a few WiFi boosters ( repeater's ) and boosted that small business up the roads Free WiFi to half the city, half the city would have free internet ( it's a far fetched idea, however still concerning for internet providers )    
Yes the difference is exactly as you stated.
One device is directly from the manufacturer and most likely a world-wide edition, meaning more language support and network band support in some case's.
However carrier released editions usually come prepackaged with all the providers software, these are usually limited to certain bands and require unlocking by the carrier to allow the other network bands... This includes any extra features they'd like to implement, like stopping WiFi hotspot and tethering.   
The story isn't much,
Carriers got scared by the simplicity of sharing Internet access to people that haven't payed for Internet... While Android made a normal business decision to allow carrier's more control over their releases of the device.     


It's all pretty straight forward and easily circumvented with additional software.