Pecan pie says "keep frozen" on the box, but the store is selling them just sitting out. Is this safe?

It really varies by exactly what it is, and what temperature it's meant to be served at.

In general, when the box says 'keep frozen', the cooking instructions assume that the item is frozen when it goes into the oven unless it specifically has multiple sets of times & temperatures.

If it's an item that's meant to be served at either fridge temperatures or room temperature, then the box likely has information about how far in advance you can thaw it (and how far in advance you need to thaw it, as it'll take a few hours if you just set it out).

Searching for that specific item's reheating instructions led me to a site that I wasn't familiar with: https://www.directionsforme.org/item/1892860

... but they have on there (with my adding some bold to highlight) :

Preparation instructions: Serve at room temperature or warm for best flavor. Pie can be thaw for up to 28 days. Ready to eat. Remove pie from carton. Thaw at room temperature for 3 1/2 - 4 hours or refrigerate overnight. Reminder Freezers, refrigerators and ovens may vary. Adjustments to time and/or temperature may be required. For a flaky crust: preheat conventional oven to 350 degrees F. Remove pie from carton. Place pie in center of oven on baking sheet and heat as follows: Conventional oven: Frozen pie - Heat for: 15-20 minutes - Let stand for: 5 minutes. Conventional oven: Refrigerated pie - Heat for: 12-15 minutes - Let stand for: 5 minutes. Conventional oven: Room temperature pie - Heat for: 10-12 minutes - Let stand for: 5 minutes. Or. To microwave: Place slice on a microwave-safe dish and heat at medium power as follows: Microwave oven: Frozen slice - Heat for: 60 seconds - Let stand for: 1 minute. Microwave oven: Refrigerated slice - Heat for: 35 seconds - Let stand for: 1 minute. Microwave oven: Room temperature slice - Heat for: 20 seconds - Let stand for: 1 minute Refrigerate or discard any unused portions. Store frozen, refrigerated or at room temperature. If purchased thawed, we recommend that you consume or refreeze within 24-48 hours. If you choose to refreeze the product, understand that it may not provide optimal taste performance.

If you bought them frozen, you can keep them frozen, possibly for months, until you want to eat them.

If you bought them already defrosted, you should eat them within the next few days.

You could refreeze them, but refreezing often affects the texture of many foods (the package will usually say "do not refreeze").