I like to play at lower limit tables too, even if I'm betting higher than the minimum. (I usually play at $5 tables, and increase my bet to $10 or $15 a hand if I get ahead. My largest bet EVER is probably a $25 bet then double-down, so I'm not a high-roller at all!)

Having said that, the Horseshu probably had a policy of having at least 1 $25 table at all times. When demand was slow, like mid-week, if only 2 tables open, that means 1 $5 and 1 $25 table. At busier times, like weekends, I assume they scheduled more dealers to work, up to the 6 tables on the floor, but with at least 1 $25 table at all times. The mix of the other tables would be based on the level of player they were trying to attract (or not attract). Like on the LV strip, the high end casinos will not have any $5 tables, or perhaps 1-2 tables as an accommodation only. For lower limits, it's either places like IP, Slots-a-Fun, etc.

Just like restaurants have different price-points, from fast food, to fast casual, to the fanciest places, casinos tend to set their game & limit mix according to the players they're trying to attract. This extends to their entertainment mix, room prices, restaurant mix, etc. It's all a package.

One final comment on why the casino prefers to keep an empty table at $25 rather than change it to a lower limit and fill it up -- they want to be sure that, if a $25+ level player does show up, he/she has an immediate seat available, or even 2-3 seats if spreading bets across several hands. This is because of the economic concept of "opportunity cost", IMO. See definition at:
http://en.wikipedia.org/wiki/Opportunity_cost

Having said all that, it is frustrating to wait for a seat when higher limit seats are available.

If you are coming to the convention at the South Point Casino in June, you'll find $5 tables without a problem. Also, some good selection of slots.