Hiya,
The 'premium' services have a Service Level Agreement ("SLA") that states the service will have a certain level of reliability or uptime, i.e. time whereby the server is functioning as it should and servicing requests properly. Should the SLA be breeched, the client is typically entitled to an amount of compensation, subject to the terms of the contract/ToS.
The free services do not have such an agreement. (and if it did there's no point as there is no payment mechanism in place)
I hope this answers your question.