I suppose it all depends on who set up the account in the first place, if they did all the ground work and set up the campaigns, and ad groups then they probably don't have to let you see it. I would rather have my account set up in house initially then grant access to third parties who then get tagged as an editor etc, that way you remain as the account holder.
I do not think it unreasonable that you have asked for evidence of what they are bidding on, negatived keywords, search terms, ad trials, optimisation etc. It is all basic data that can be shared. Like Egol said, look into your contract and see what they should be divulging via their reports etc.
Finally....
Are they proving their worth? If not consider outsourcing to someone/ some agency that can or is willing to share, build a relationship and improve.