Here's the official bit from
Amazon Payments Terms of Service (bold is mine):
6.2 - Suspension or Termination by Us. We may suspend this Agreement or terminate this Agreement and close your Payment Account for
any reason at any time upon notice to you.
6.3 - Effect of Termination. We will not be liable to you for
compensation, reimbursement, or damages of any kind in connection with any termination or suspension of the Service.
MTurk's TOS also has a relevant bit:
4b - We reserve the right to cancel your Payment Account
for any reason.
4d - Restrictions and Limitations. We reserve the right to terminate or suspend any Payment Account, or to delay the availability of any Prepaid HITs, transfer or disbursement of any amounts, in each case
for any reason in our sole discretion, including, without limitation, if we believe that a Requester or Provider is in violation of this Agreement. We reserve the right to restrict the transfer to Providers of any amounts held in a Requester's Prepaid HIT account for such time as we reasonable deem necessary to protect us or others: (a) if we are subject to financial risk, (b) if Provider has violated any term of this Agreement or the Policies, (c) if any dispute exists involving Provider's Payment Account or involving the Services provided by Provider, or (d) in connection with fraudulent, abusive or unlawful activities as determined by us.