reasons i disagree with natasha being on team tony:

• her speech at the end of catws in which she says “you’re not going to put any of us in a prison. you know why? because you need us” doesn’t sound like she’s all for superhero accountability
  
• she’s aware of hydra’s presence (possibly in the government) and how that could put registered people at risk
• she’s shown little or no trust to the people on team iron man (as opposed to the level of trust she has with clint/steve); trust is a common theme in natasha’s characterization and is an important part of her relationships

not a reason i disagree with natasha being on team tony:

• i think she should “follow steve around”