I think it's just a disclaimer. Other sites say to soak in the salts. There are cases in which it is frowned upon and should be discussed with a doctor.
The thing is Epsom salts can be used orally as a laxative. It's my understanding that it's saying to not do that. I've taken a few Epsom salt baths for soreness and I say go for it.
I'm pretty sure it's just one of those "you can't do this because you are pregnant" disclaimers. I Googled multiple things last night- not one pointed towards it being unsafe!
Re: Epsom Salts
DS: Born 5-17-16
Me: 31
DH: 29, SA - Great
Married: June 12,2011
TTC #1: 1/2014
Diagnosis: Hypothalamic Amenorrhea
Treatment: Clomid: 50mg, 100mg, 150mg - not successful and not monitored
Menopur 75ml (upped to 112.5ml), Ovidrel, & IUI IUI #1 8/31/2015
9/15/2015: BFP HCG - 400, 9/17/2015: HCG - 827, 9/21/2015 - HCG 3,327!to me. You're fine!