Who said you can't just because Alan Gow doesn't let you? Modding is not illegal in any country of the world, and a free mod cannot be stopped. If it was a commercial product or a product that earnt money, then yes he has power to stop it. But if he tries to stop a non commercial, non profit, freeware mod, he will hit his face very hard in the courtroom and wonder why he even bothered. The licence to BTCC covers items that are commercial and non commercial and MAKE A PROFIT, not items that do not make a profit in the non commercial. Modding is made for this purpose, so people can replicate seasons. This is what modding is and why it was formed. If mod groups weren't **** scared to take these companies on, you'd have more mods out. But then, thats why you have MMG to, cause we ain't scared.
An infringement of trademark/copyright is an infringement nonetheless. Whether the project is for profit or not has no bearing on it. Ultimately, Alan Gow's TOCA company owns the commercial rights to the series and if you were to release a mod of that series, you wouldn't have a leg to stand on in court.
There are ways around it of course; just see the BMS in the STCC game, but I suggest you seek legal advice if you're hoping to release any such mod.