Just use that function and see if you are being scammed or not. If you think they are making you lose against the provably fair function then they will be surely going down in a day, everyone will talk about how freebitco.in doesn't follow their own provably fair and scams peoples money from them and everyone will leave the website withdrawing their money. Moreover, that is not happening, do you know why ? Because, they are not. They have a dice game where the game is provably fair and can be PROVED (as in provably) that they are not scamming anyone. You are just unlucky and you want to put the blame on the website.
It's one thing to "say" you use provably fair and quite another to "actually" use it. Newsflash, there ARE people who have caught them changing seeds in the Scam Accusations thread (https://bitcointalksearch.org/topic/freebitcoin-provably-cheating-5045277). It's also not necessary to prove anything by use of seed manipulation either. Large samples can prove the same thing without the need for showing seed changes (which I've done that also confirms what those with changing seeds have claimed). And it certainly wouldnt take "a day" to "go down". There are some incredibly stupid people out here (this is where you should see yourself in the mirror if you're remove that duct tape off your face) and this thread is proof of that. Scams last 100x or more longer than they should. Take Bitconnect or any of the pyramid smart contract games for example (P3D comes to mind). They last plenty long enough and are very good proof that just because something is a scam doesnt mean it's going to end in less than a day. It also doesnt mean that there are not going to be idiots who claim that it's legit even when it's OBVIOUS that it isnt. Just because freebitco has been around for a long time doesnt mean it's legit. It just means that there are a lot of incredibly stupid people who use it which in turn allows it to continue running. And I consider anyone who uses "longevity" as a means to legitimize something as one of those stupid people because longevity doesnt mean something is legit. The clients of Bernie Madoff could tell you all about how that assumption worked out for them.