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It's to stop people booking everything up when there's no guarantee they'll even make their targets and depriving other racers of services.
I find that explanation desperately unconvincing. I suppose it depends where the race is taking place, but unless it's in the absolute arse-end of nowhere I really can't imagine that ultracycling racers on that particular race are going to make any appreciable difference to the availability of accommodation. When Cycliste and I did TPBR we stayed in a hotel every night, and on absolutely no occasions were we sharing the hotel with another racer. OK, we weren't exactly at the pointy end of the field, but they wouldn't be in hotels anyway.
Brommers-
TPBR is semi-freeroute so less of an issue. Fixed route races means all racers will be in the same towns. Some key towns on the TABR route have one place to stay. It does have an impact whether you care to admit it or not and even if it has zero impact it's against the rules so you're a dick if you're booking services outside race time.
hippy
Yes. Most races stipulate no booking stuff before race starts and/or no booking stuff outside 24hr period in the race. It's to stop people booking everything up when there's no guarantee they'll even make their targets and depriving other racers of services.