So, all they can prove or show is that you hit "value" lines or "stale" lines?
Maybe it's better to take your bonuses with the "sharp" bookies and play the "soft" bookies straight.
They can tell if you hit value prices just like they can when you're not betting with a bonus, but them identifying you hitting value prices is not grounds for them to confiscate bonus and winnings.
You'll be hitting value prices any way as that is an occupational hazard of arbing so whether you're betting those value prices when you have an active bonus or if you're betting with bonus funds, that makes little difference.
I have always taken bonuses when available and I've never felt that taking the bonus meant it took a shorter time to get limited as opposed to if I hadn't taken the bonus and as long as you use reputable bookies, you shouldn't get into any problems with bookies voiding your bonus because of arbitrage play etc so I feel that not taking bonuses is just leaving money on the table unnecessarily.