Restaurant etiquette

I lunch at the Milk St. Cafe regularly. It's a restaurant with a cafeteria-style line and trays to take to a table, with staff busing trays when customers are done. They're only open for breakfast and lunch.

Last week, I noticed a man sitting nearby eating his own lunch from home: not only was it in a brown paper bag, it included items that were not likely to be kosher (gummy candy). He hadn't bought anything from the restaurant, nor was he meeting anyone for lunch.

This week, a suited man came in with a bag from a restaurant up the street, chose a table, and ate his meal. Again, nothing was purchased from Milk St., nor was he meeting anyone. I noticed as I walked by that the restaurant he'd gotten his lunch at was quite full; apparently he'd decided that since there wasn't a table available there, he'd walk down the street to another establishment and eat there instead.

Am I missing something? I know it's winter, but it still doesn't seem appropriate to use a restaurant's table (at noontime, their busiest time) without at least buying a drink. Is there some circumstance that would make this acceptable?

For Cantabridgian koshertarians

There's a new little kosher mart at MIT. The prices don't look particularly good, but in a pinch, having something there is useful. (Though the listing for Elite brand "Kid Flavored Gum" is a little worrying...)