The data the NSA collects itself is only from sources outside the USA
That's clearly not the case.
The systems where the NSA does its own collections of cabled communications are actually constructed outside of the USA, they have a some interesting structures built in Canada to do that for them. Not sure where the system for the collection of the satellite communications are. The great bulk of those communications are with people outside of the USA, however, sometimes the communications companies (especially ISPs) direct a message through circuits that leave the USA and return because of the traffic levels, and such cases also get picked up by the NSA. All that information is sent to the NSA computers for analysis. The NSA also gets a lot of data sent to it that was collected by the associated agencies in other countries like the UK, NZ, Australia, Canada, etc. (do a wikipedia check on ECHELON for more details). Thus NSA is used to incorporating data from other sources in their analysis process. Thus, when they enacted the Patriot Act they told the internal USA agencies collecting data to send it to the NSA for analysis alongside what the NSA already have - and that's where the data management program PRISM comes into the picture.
What people get confused about is the data collected inside the USA isn't collected by NSA or organised for collection by NSA, but is collected by various local, state, and federal organisations and piped over to the NSA for analysis. The process works by FBI etc. establishing a collection point and then installing gear to ship the collected material direct to the NSA - thus the FBI collects and the NSA analyses. It's like where someone owns a few oil wells who pump the oil, then ship it to a refinery owned by someone else, the refinery owners don't drill or control the well, they just process the oil produced by it.
Your comment on safeguards is very true, but there are already many safeguards in place on what NSA does and doesn't collect itself. However, they have no application or control over what the other agencies collect and ship to the NSA for inclusion, and that's where the problems are, and where better safeguards are needed.
Mind you, the NSA computers process so much data that over 99% of it is dumped direct through their filters without any record being kept it exists at all. Before it gets to the real analytical stages it goes through several automated screens, and only if it meets the criteria for deeper checking does it end up being closely looked at. Those early screens look for certain catch words and phrases only, and later ones look for patterns in what has been bumped up the line. The process of winnowing continues until something is passed up to the point it needs to be looked at by a human, and that is something along the lines of 0.0000000001% of the material initially collected.