I imagine Morecambe will say this was not intended for fans of the opposition club, like it is not intended for fans of the home club. It would be a crazy thing to let Mansfield fans (who would have been coming anyway) in for £10 and not Morecambe fans who are not season ticket holders. Quite a blunder not to put that in the small print though. Someone asked on the Morecambe FC facebook page if it applies to Mansfield fans, but there is no reply.
I'm sure it is an attempt to attract fans of other clubs who would not ordinarily be coming to this game.
There is a similar thing at cricket. If you are a Kent member, you can go and watch Sussex at Hove for just £6 but NOT if they are playing against Kent.
That's not to say they won't honour it though. Maybe they will.
"Four points clear as Lincoln are McCaffreyised", CHAD headline, April 1975