I think this depends on the family.
My parents are very equal with their children and the spouses. Down to the last penny equal.
My in-laws... are not. However, I don't begrudge them. That's life. The husband is their kid. They might want, from time to time, to do something special for him. Doesn't bother me in the least.
I should say, though, that my husband is a generous person. When that gift happens to be monetary, it goes into our shared account. So, even if his parents didn't plan it that way, it ends up going to both of us in a way.
As to what your in-laws did. That gift to your husband was not a "hostess" gift. It was "here, son, you're our son and here's a random gift" gift. The "thank you for having us at our house" gift was the breakfast for both of you.
What would have made you happy? If your husband had gotten the $300, but you had gotten the $100?
Since it's not likely that your in-laws will change this behavior, what if maybe your husband were to take some of that gift card money and use it on both of you for something? Though, that depends entirely upon how you and your husband divide gifts and money.
Again, since we can only control our own behaviors, would you feel better if you only gave hosting "50%" instead of "100%" the next time your in-laws come? Or if you divide up the chores even more with your husband when his parents come. He has to do more work when his parents are over, because they're his parents?
Final verdict: Your in-laws really didn't do anything wrong. The gift was a random gift to their son. Their thanks to the both of you was in the breakfast out. I'm not going to say that's always a comfortable split for every family, but it's not rude. It's just different. Maybe the best you can do in this situation is remember it for if you ever end up having a son-in-law or daughter-in-law.
The only thing that's wrong is if these gifts are going to get between you and your husband. I'm not saying they are, I'm just hoping not. If you feel resentful towards your husband, then maybe an "I feel that..." chat is in order. Just so that the two of you can work out how to handle unequal gifts as a couple. Because, again, it's unlikely the in-laws will suddenly change, and I don't think it's a good idea to ask them to change or anything like that.
ETA: Extra thoughts.