That's correct. Selling sex is not illegal. You can get charged for running or being found in a place that is used for the sale of sex (bawdy house), living off the avails of selling sex (pimping), communicating in a public place for the sale of sex, etc.
That's why incalls (and high mileage MPs) are riskier than outcalls for customers. We run the risk of being caught up in a raid and charged as "found ins" at a bawdy house. (The other criminal charges don't really apply to customers.)
It's been discussed elsewhere before but worth mentioning again:
1. If you're worried about safety, stick to the top spas that are well run and know the rules. Avoid the sketchy spas that arent properly licensed and dont care whether they are busted.
2. If you do get caught up and are charged as a found-in, STFU. LE or the crown may lean on you to testify on what happened in the spa (to help prove their case). They will tempt you with pleading out or some offer. Talk to your lawyer before you yap. if they can't prove the bawdy house charge, your found-in charge is moot. Don't help them.