I knew it was going to be a rather short single player campaign. Honestly, I would think that it would have been common knowledge, or at least, expected that the campaign would last 10 hours or less. It was designed to tell a section of a story, which it did, but the main selling point, I would think, would have been the MP, which is why I bought it now instead of waiting. If you seriously expected FOC to be like 20-30 hours or higher just for the SP, then I gotta say that it is really just on you. WFC was the same way - decent but short SP and a fun MP. Very few games with a SP campaign and a MP section that come on a single disc are gonna have long campaigns.
If you want the SP portion’s length to justify buying a game at full price, then I’d steer clear of any game with MP. Personally, I bought it for the MP and the time spent playing online will more than justify the cost, so I have no complaints about the shortness of the campaign.