Why? Because a peaceful community center set up by people who are meek and good in heart will be commandeered by people who are neither meek nor good. What are those weakings going to do when some new Imam is sent to "take care" of the facility? Is there suddenly a shortage of mullahs who dislike the USA?
Besides, a community center will be a slippery slope. There will be so much space inside the facility that it's inevitable that some rooms will be allocated for prayers, thus making it a de-facto mosque. It's marginally possible to stop the project now, but how do you forbid specific use of rooms within a completed private building, years down the road? There is no way to forbid that.
There is no URL to the source, so I have no idea who wrote it, but such a moderate muslim should simply say "we sinned, and as a penance we give up the right to build a mosque here." This seems to be very obvious.