Given the Teamsters are endorsing these views, I wouldn’t be so quick to jump to conclusions. Amazon is building an internal UPS competitor. UPS may simply not want to over-extend itself.
If I were to wildly speculate, Amazon is happy to give shipping partners as much low margin volume as they're willing to take. UPS has higher labor costs because they have union workers, while Amazon does not. To your point, the Teamsters are fine with this because their members have a contract and labor protections from arbitrary cuts. ~70% of UPS' 443k employees are represented by the Teamsters' Union, per UPS' website.
An average UPS driver pulls ~$100k, with some getting close to $170k. Amazon delivery network humans are not approaching this comp and benefits. That is a material component of the profitability spread imho.
