Yeah, they aren't good for your tank. If they don't realize food is coming out the food will sink to the bottom, rot and cause ammonia which will kill your fish by the time you get back.
If your fish finds out it is food, he/she is going to keep eating until it's all gone. Bettas are what you call "eat all you can now because we don't know when our next meal is" so they will eat and eat until they kill themselves. Overfeeding is one of the most common ways fish die. Plus, the food in those things is junk. Bettas are meat eaters, therefore need a pellet with at least the first 3 ingredients some sort of meat. So even if your fish does miraculously survive, it's not helping him/her in any dietary sort of way.