Well....no.
There were many strong 15-rated films that were touch and go in the first place - and were only allowed a cinema 15-rating due to the fact kiddies definitely weren't allowed in to those screenings by law. So in a way, they were being quite generous to opt for a lower rating at cinema release to allow 15yo's to see it.
And back in the day, 18-rated video tapes were generally kept out of reach of young children a lot more than 15-rated videos. It was a sound theory, although it's a bit of a moot point these days, I doubt the BBFC bother to even think of upgrading 15's for those reasons any more...
As long as you can see the context of the medium and its availability, it made sense I think.