Miners do not have to back SegWit if only a softfork is required. Once a BIP is published and the core developers reach agreement it will be merged into Bitcoin Core and miners will have to live with it.
The only way to ensure a soft-forks success is to backport the change, as some miners use older versions of bitcoind. They can continue to use the older version until quorum is met. If not, they might produce invalid blocks (according to the 95% of the network prepared to adopt the change)
It's hardly forcing miners, when they're responsible for signalling support.