Maybe I'm ethically challenged but I don't see a problem with it. I imagine the cost of the coffee vs the cost of gym membership is de minimis. I did some quick calcs and it looks like the rocket fuel I drink is around $0.10 to $0.20 for one of those "16 to 20 oz big gulps".
If they come in every other day during the month and fill up on coffee at each visit, then they are perhaps consuming $3 per coffee per month. Maybe $5 if you throw a little sugar and creamer in.
What is the ethically allowed serving? 1-2 6 oz cups per gym visit?? Cost will be very similar, and you consume one of the cups they are paying for (at least a couple pennies in value). Say the in house drinker consumes $3 worth incl. cream, sugar, and disposable cups.
So the Mr Big Shot has managed to eek out an additional $2 in value from the gym each month. As compared to a ~$50 membership? Not a big deal. All types of consumers patronize facilities like the gym, and some consume more resources than others. Maybe some shower there, others don't. Some use lots of toilet paper or hand soap, others use none (they should really wipe though). Some use lots of towels and generate more laundry, others pack their own. Some may only use the gym a few times a month.
I would even hesitate to classify a 16 to 20 oz container as a big gulp. I think that is the standard size for travel mugs. When I read Big Gulp I figured somebody had their 64 oz trucker mug from the gas station in there emptying the coffee carafes. My mistake!
I imagine the patrons that take the coffee are in a hurry, and value the 5-10 minutes it saves them versus waiting in a drive thru or stopping at a coffee place on the way to work or wherever. If they had unlimited free time they could probably sit down for a spell and sip on a 6 oz styrofoam cup or two of the brew. Regretfully people have busy schedules, and after their self indulgent work out they are probably off to the next agenda item of the day.
I wouldn't hesitate to bring in my (roughly) 14 oz travel mug and fill it about 2/3 to 3/4 full of the gym's delicious brew if I had stopped by the gym on the way to work. It would be a convenience that might make me prefer one gym over another since the time saved in coffee prep or buying would partially offset the time spent working out. I like a little more than 6 oz of coffee but don't really want 16-20 oz (probably isn't good for you). And I like the coffee cool enough to drink comfortably. As a result the 6 oz styrofoam cups wouldn't serve my needs well. I'd rather take my 6 oz allotment and let it sit for a little while to cool down. Which lends itself to the to go travel mug.
If the gym really wanted to control their costs on coffee, they could put up a sign that said "for consumption in the gym only" and/or "no travel mugs" or "use provided cups only" or "no to go coffee". I doubt that is their intent. It is a value added service that is convenient for patrons and they probably aren't bothered by the pennies they are losing on the Mr Big Shot Big Gulpers.