I don't think UASF can be categorically labeled an attack. Otherwise it would mean that miners largely control bitcoin- they alone get to determine whether or not a soft fork is done, despite only being a subset of stakeholders in the Bitcoin ecosystem. The possibility for a UASF spreads out that control and puts a check on miner incentives. 

 I agree with this! 

 Miners largely control Bitcoin. 
Not sure what world you live in, but in this world miners have the ultimate saying on what is a block and what is not.