It's not just that, sadly. They think somehow trout will escape Fairmount, swim down the little muddy creek behind it, and enter the Santa Ana River, thereby potentially diluting the genetics of native steelhead in the river.
Note the word
potentially. There probably hasn't been a trout in the Santa Ana river in the Riverside to Anaheim stretch in the last 30 years. The river is ridiculously shallow, fast, and very warm, with most of the water from wastewater treatment plants. It's basically the IE's version of the LA River. Can you imagine trout thriving in the sewage-choked run off of the LA River? Yeah, thats exactly how much of a stretch this concern is. Rainbow trout spreading and breeding in the lower Santa Ana, right alongside carp and channel catfish.
So, as appears to be often the case in SoCal, an angling opportunity (stocker trout in Fairmount) is lost due to the extremely unlikely chance it might interfere with a species that basically doesn't exist in the watershed.