That was me, and here's a simple answer. It's even moar complex than this.
The census serves more than just one purpose. It's main function is to determine the population count per state so that the apportionment of Representatives in the House is updated…but that's not its only function.
The population data from that also determines (for much of the following 10 years) how money appropriated by Congress will be spent. The apportionment principle also has some application to whether California gets more, or less, aid over the next 10 years than, say - Texas. Population percentages vary, so do the percentages of money divvied up.
POTUS just tweeted something to that effect - all states are not the same.
There's a few other effects that the census has on various programs, but it also has a HUGE effect on city/county determinations for Planning Departments, etc.
For instance, if there are 10 restaurants in your small town and 5 more want to open up, do you have the land available for it? Will the population support it, or will it drive 8 of the 10 existing restaurants out of business (and thereby reduce the overall amount of food businesses from 10 to 7)?
If that sounds anti-free market to you, it sounds that way - but keep this in mind, those existing 10 restaurants all have people, families, lives, etc. So the numbers affect the overall balance.
Doncha think it would help if the numbers weren't fucked because people didn't want to participate?